For artists, color theory is a guide to mixing colors and is especially useful for achieving goals such as a special effect or a certain pigment. However, there’s actually a lot of science in colors. For anyone who’s taken physics 214 or any class that works with photons, the electromagnetic spectrum should be familiar.
Quick review: The spectrum gives the range of wavelengths of visible light, with longer wavelengths corresponding to colors in the red-orange-yellow part of the rainbow (ROY) and shorter wavelengths corresponding to blue-indigo-violet (BIV). Green wavelengths fall more in the middle of this spectrum.
The blue and green swirls in this optical illusion are actually the same color.
An interesting fact: caramel colors, like food additives for candy and the color of beer, have their own hue index separate from the rest of the color spectrum. All photons travel at the speed of light, c = 3E8 m/s, so longer wavelength photons have a lower frequency, and shorter wavelength photons have a higher frequency according to the equation v = f*λ.
How does an object have color? This is a little counterintuitive. The material actually reflects the wavelengths of the color that you see, and absorbs the rest of the spectrum. Black is the absence of light, so black materials are absorbing all the photons that collide with them (this is why black shirts feel warmer). White is the combination of all the wavelengths, so white materials are reflecting all the photons. A green shirt absorbs all wavelengths except green.
The atmosphere works a little differently. White light from the sun hits the outer layer of the atmosphere and experiences scattering. The gases in our atmosphere scatter light in the BIV part of the spectrum more easily than ROY light, with violet being the easiest. For most of the day, BIV wavelengths are scattered the most, giving us a blue sky. At sunset, the sunlight has to travel through more of the atmosphere to reach us, which results in increased scattering of ROY light, making us see red/orange.
Light passing through a longer section of the atmosphere experiences more scattering.
In color theory, the well-known primary colors red, blue, and yellow are considered the artist’s primary colors. Secondary colors are colors created by mixing two of these primary colors, i.e. orange, green, and purple. Complementary colors are two colors that when mixed together create gray, or basically cancel each other out. These complementary pairs are red and green, blue and orange, and yellow and purple. You can see the pattern here: green, for example, is a mix of blue and yellow, aka two primary colors, added with the third primary color, red.
This method is called subtractive primary colors. Printers use a version of this method with magenta, yellow, and cyan ink. Digital colors are created using a different method called additive primary colors, in which the three primary colors are red, blue, and green. This is based in light mixture instead of pigment mixture.
Isaac Newton's color wheel.
Those of you who have coded colors will be familiar with this. For the RGB method, you can determine the hue region (hue = the actual color created) by looking at how much of each primary color is present. For example, B>G>R = cyan-blue region; G>R>=B = yellow-green region.
A lot of optical illusions are based off a section of color theory called Chevreul’s Law, which basically says that placing contrasting (aka not similar) colors together will cause the colors to appear tinted with each other’s complementary color. For example, placing a yellow cloth on a red background will give the cloth a green tint, because green is red’s complementary color. Similarly, putting a saturated (higher intensity) color like red next to a neutral color like gray will make the gray appear tinted with green.
In the swirly illusion shown above, the blue and green swirls are actually the same color, but appear to be tinted differently because of the placement of purple and orange.