About Color

Sir Isaac Newton was the first person to study light and color scientifically. When sunlight passes through a prism, it separates into seven distinct colors. But when one of these colors is passed through another prism, it remains unchanged. In addition, passing the seven colors through another prism combines them back into the original white light. Based on these observations, Newton was the first to show that rather than the prism changing the color of light, white light itself consists of seven colors (the spectrum). We see color as a result of how much of each of the seven colors of the spectrum is absorbed or reflected.

Under the sea

The blue sky as observed from an airplane

It was discovered that the sky's blue color results from the scattering of light, not by dust and water droplets in the air, but by the air itself (air molecules). According to this theory, known as Rayleigh scattering, the scattering of light caused by particles smaller than wavelengths of light is about 10 times more likely for light with a short wavelength (blue) than for long wavelengths (red). This is why the sky appears blue.
The blue sea, however, is a result not of Rayleigh scattering, but of water absorbing the red component of light.

The blue sky as observed from an airplane

Under the sea

Mie scattering refers to light scattering by particles of about the same size as wavelengths of the light. The result appears white, since the light is generally scattered evenly and independently of wavelength. Clouds in the sky appear white because they scatter sunlight evenly.

White clouds in the sky

Color mixture in light follows the principle of additive color mixture. The three primary colors of red, green, and blue can be used to reproduce a wide range of colors, and can be combined to form white. TVs and other displays are common examples of the use of this principle to produce colors.

Color mixture in light is an additive color mixture. When all three colors overlap, the result is white.

White pearl is produced by mixing red, green, and blue (RGB) flakes

Subtractive (left) and additive (right) color mixture

Color mixture is based on the principle of subtractive color mixture, in which the three primary colors of cyan, yellow, and magenta are mixed to reproduce various colors. When these three overlap, the resulting color appears black. If you look at printed materials through a magnifying glass, you can see that the colors consist of tiny dots in the three primary colors.
Paint mixtures are based primarily on the principle of subtractive color mixture, like the mixtures used by artists. The microscopic luminance of lustrous pearl colors often used in automotive coatings are, however, based on the principle of additive color mixture, just like light. For example, mixing red and green pearl colors produces a yellowish silver color, an overlap of shining red light and green light. In this way, the principles of color mixture are skillfully used to develop colors in the field of color development.

Subtractive (left) and additive (right) color mixture

White pearl is produced by mixing red, green, and blue (RGB) flakes

We use the various types of light all around us depending on the purpose. These include not just light bulbs, but LEDs and lasers. While sunlight includes light in all of the visible wavelengths, some light sources emit larger amounts of light in specific wavelengths compared to other light sources. For example, things look quite different under a daylight fluorescent lamp compared to a mercury-vapor lamp. This is an example of how color varies with the amount of red included in each light source. Since a mercury lamp emits almost no red light, it makes bananas and oranges appear to be the same color.

Structural color refers to colors that result from various optical phenomena, such as light interference and scattering, due to microscopic structures of the same size or even smaller than light wavelengths. A well-known example of structural color is iridescent coloring. Structural colors will not fade unless the structure itself from which it results changes. On the other hand, changing the incident angle of light or perspective can result in different colors because the light wavelengths that match the periodic structure of the object will change.

A morpho butterfly's color changes when viewed from different angles

Electromagnetic waves at wavelengths visible to the human eye are called visible light. Those of wavelengths shorter than visible light are called ultraviolet, and those of longer wavelengths are called infrared light. Ultraviolet rays are high-energy rays which are even able to harden resins. UV light has been used to develop technologies like photo resists to form electronic circuits and UV-hardened coatings that harden instantly without heat.
Infrared rays, on the other hand, have heating effects. Heat-blocking coatings are being developed to reduce heat generation/retention on rooftops and streets by efficiently reflecting infrared light.

(1) Normal coating: high temperature (red)
(2) Heat-blocking coating: low temperature (yellow)

An aggregation of transparent particles or fibers will appear white because the surface reflects light in all directions. The greater the variation in angle of refraction and the smaller the particle size, the whiter it will appear.
When materials like sugar, salt, and starch are viewed through a microscope, we see that they are translucent crystals. Starch appears whiter because its particles are smaller.

Starch→Sugar→Salt

Titanium white is a white pigment whose particles are produced at sizes at which they reflect (scatter) light most efficiently. Since its particles are extremely small (about 0.2 microns in diameter), it appears white even when viewed through an optical microscope. But it looks translucent under reflected and transmitted light viewed with an electron microscope.
It’s widely used as a basic coating material, not just for automotive coatings, but for coatings for bridges, buildings, and home appliances. It’s also used as a photocatalyst.

Viewed through an electron microscope

Prussian blue is said to be the first artificial blue. Consisting of ferric hexacyanoferrate, this pigment has been used widely in Japan, together with indigo, since the Edo Period (1600–1868), and remains widely used in printing inks today.
Cobalt blue was developed in the early 19th century as a complex oxide of aluminum and cobalt. The later chemical synthesis of ultramarine in France made it possible to achieve numerous shades of blue. The blue we see all around us today in printed materials, plastics, and coatings almost always is produced using copper phthalocyanine pigments. Most pigments in the range from blue through green contain phthalocyanine. Phthalocyanine is also widely used in combination with metals other than copper as a functional material, in addition to its use as a colorant. For example, it’s used in solar power and medical sensitizers as an organic photoreceptor and light sensitizer.

Comparison of pigments: natural blue (left) and synthetic blue (right)

One often sees sand, asphalt, and other materials blacken as they absorb water. That’s because water restrains reflections of light on the surface so that the light permeates into the material. Permeated light is absorbed through repeated reflection, so that it cannot escape from the surface, resulting in a black appearance.
For example, a bundle of bright chrome-plated needles will appear black when viewed from above. This is because light repeatedly reflects among the needles and is absorbed by the metal.

A bundle of 1,000 needles

Carbon black is a black pigment consisting mainly of carbon from sources like natural gas and petroleum produced through partial incineration. The term carbon black as a black pigment refers to ultrafine carbon black including channel black (fine particles produced from natural gas) and furnace black (produced from petroleum; the most common type of commercially available carbon black). Carbon black can be used to produce various types of black by controlling particle size. This is because its properties depend on the size and surface properties of the particles. For all coatings, black is produced by scattering pigments in resin.

Carbon black pigment

Today, to produce the wide range of colors in multiple coating fields, Kansai Paint uses hundreds of coloring pigments, from which we choose the best ones for each application. We select organic blue pigments for individual applications from a broad range of options including phthalocyanine blue, indanthrene blue, and other pigments as they vary widely in properties and tones. Pigments used in coatings are subjected to strict testing to check and ensure quality. For example, only pigments passing accelerated weathering tests by exposure to powerful xenon lamps for 1,000 hours or longer or outdoor exposure for two or more years in Okinawa are used in actual coatings.

Pigment samples

When a metal compound containing various elements is burned, it generates flames of various colors specific to each of its elements. This is called a flame reaction and is used in colorful fireworks that light up the night sky. The phenomenon that underlines the generation of element-specific light makes it possible to analyze elements found in an unknown sample by evaluating the light generated. Kansai Paint uses ICP emission spectroscopic analysis equipment that uses plasma heat (thousands of degrees), much hotter than the flame used in microanalysis of lead, cadmium, mercury, and other heavy metals found in coatings and their raw materials.

Gas→Sodium→Potassium→Calcium→Copper

Metals both absorb and reflect light strongly. This combination of properties results in colors with a metallic sheen. For example, mercury appears silver because it reflects more than 95% of visible light, absorbing the remaining 5% or so. While metals normally are not light permeable, light can pass through extremely thin metals. A two-way mirror is one example of this property.

Reflective windows (two-way mirrors)

Since a coating's surface reflects light, a white fluorescent lamp will appear white and remain unaffected by color. Metals are the sole exception. Metal has very high light reflectivity. An aluminum sheet features reflectivity exceeding 90%. Another property of metal light reflection is that the reflected light has its own distinctive color specific to the metal, which is not the case with nonmetals such as coatings.
Another example of coloring by reflective light is the light reflected from the surface of a lens. Eyeglasses have antireflective coatings. These coatings are extremely thin, and they appear to have color, which is due to light interference. The colors change with the angle of view. Colors from metals and interference effects are used in materials for metallic and pearl coatings.

Light reflecting from metal

Silver metallic paint through a microscope

Silver metallic paint

Silver metallic colors have a bright sheen and a three-dimensional feel that makes shapes more attractive. The brilliance of this color comes from scaly aluminum flakes produced by finely pulverizing aluminum. Efforts to develop even more brilliant coatings have led to the creation of a chrome-like texture, thanks to special coatings made with ultrathin aluminum flakes produced from evaporated aluminum film.

Silver metallic paint

Silver metallic paint through a microscope

The layers of a pearl consist of bricklike crystalline layers of the protein conchiolin and the calcium carbonate aragonite. Even at less than 1 mm thick, over a thousand layers can be found, with each layer reflecting light differently. The distinctive brilliance of a pearl, which appears to be illuminated from the inside, results from these multiple layers of reflection and light interference. This is referred to in the industry as a pearl color.
* Multiple reflections: Repeated reflection of light between two facing surfaces

Interior of a pearl

Structure of a bubble and liquid film and the principles of light interference

Life of a bubble

Interference colors refer to the colors produced by the relationship between light and a thin translucent coat, something seen in the infinitely varied colors of soap bubbles. The coat starts thick, and as it thins over time the colors change. This is attributable to the wave-like aspect of light and its refractions that vary with wavelength. Technologies developed in the 1960s for coatings used titanium dioxide on transparent flakes from pulverized natural mica, which reflect light with high refraction. Also developed at the time were ways to generate various interference colors by changing the thickness of the titanium dioxide.

Life of a bubble

Structure of a bubble and liquid film and the principles of light interference

A glass of wine and a color reproduced from it

In the field of automotive color development, which represent the state of the art of coating color development, color texture refers to the luminance, depth, transparency, and luxury feel of a color. Efforts are ongoing to develop new designs while adjusting the three color elements of hue, brightness, and saturation, in addition to texture and minute color variations.

A glass of wine and a color reproduced from it

Gloss, or specular gloss, is an important aspect to note when checking the quality of a paint or coating. It’s also a factor when testing the durability of a coating, which is determined by tracking how long the initial gloss is maintained. Just as color depends on the relationship between light and the object, gloss depends on the optical qualities of the object's surface. While gloss refers simply to the phenomenon of light reflecting from the object's surface, it also has deep ties to color. Like color, it’s an element that affects how an object appears.

Glossmeter

Since light reflection depends on the angle, gloss values from specific angles, like 20° gloss, which is close to perpendicular, and 60° gloss, are used in gloss evaluations to judge surface smoothness. In architecture and other fields, 85° gloss may be used as well. The matte surfaces used in interiors and other applications must be free of gloss, but when one looks from a direction that approaches the parallel to the wall, surface gloss increases. This is why it’s important to evaluate gloss at the angle of 85°.

Viewed from close to parallel, the same matte wall surface appears glossy.

Kansai Paint has a number of outstanding gloss control technologies. For example, the industrial coatings applied to precoated metals are quickly hardened in high-temperature kilns. The coatings are designed so that uneven surfaces will form as the coating surface contracts during this process, resulting in matt surface coatings that are highly durable and suitable for processes like curving. These are used as materials for home appliances and other products and for metal siding.

Left: Neomatte surface Right: Siding

Color popularity analysis involves graphing the history of color trends. Kansai Paint uses this analysis primarily in its development of automotive body colors. The analysis reflects cultural aspects of color and trends that vary by country and region.

Body- color survey of vehicles on Tokyo streets (June 2019, Tokyo, Japan)

Kansai Paint has assembled a digital library of more than 40 years of color information, including information on mixing and other technologies, optical measurements, and even emotional information. We use this library as a digital tool for color development.
For example, when developing a new color like "a glass of red wine" as described by designers, we search for the one that exemplifies their vision of the color in question from tens of thousands of past color development projects in the library.

Examples of using the digital tool to search for colors approaching designers' visions

In efforts to produce paints, toning refers to the action of mixing various primary colors to achieve the exact color sought. This process is handled by toning engineers with expert knowledge of colors and materials. Today, color theory and computers are used to determine mixing formula based on calculations of light absorption and scattering when mixing various pigments. The technology of computer color matching (CCM) is combined with automatic measurement equipment for in-can toning, which makes it easy to tone paint in a volume as small as a single container can. Our toning technologies will continue to evolve in the future.

In-can toning machine