Color theory can be confusing.
Let’s start with the way you look at color with your eyes. As it turns out, your eyes have three different cone photoreceptor types, one each for red, green and blue light. They interpret color in essentially two ways. One way is light that you directly perceive from a light source such as your computer screen. The other way is light reflected from an object such as a printed page or a painting, Both of these ways of perceiving light are interpreted by the cone photoreceptors in your eyes. However, emitted light is considered additive, and reflected light is considered subtractive.
What does that mean?
For example, most computer people would tell you that the primary colors are red, green, and blue which is where the term RGB (Red-Green-Blue) comes from. In fact, any given color is defined in hexadecimal as a number between #000000 and #FFFFFF. The ‘#’ sign in front of these numbers indicates that the number is expressed in hexadecimal, or base 16. Base 16 uses the digits “0, 1, 2, 3, 4, 5, 6, 7, 8, 9” and then adds “A, B, C, D, E, F” for a total of 16 digits. The first two digits of the hexadecimal expression of a color represent the amount of the color red from #00 to #FF or if you prefer decimal numbers, from 0 to 255. Similarly, the next two hexadecimal digits represent the amount of green and the last two hexadecimal digits represent the amount of blue. Because each of these three colors has 256 possible values, the combination of the three colors is the product of the total number of values in each color. This means 256 x 256 x 256 or 16,777,216 possible colors. With those possible values, they define all the colors you see on your computer screen. So that is the end of the story, right?
No, but we will get back to RGB in a moment.
First, if you were to turn to your trusty source of all information, the Internet, you would find that a search on Primary Colors would return Red, Yellow, Blue, not Red, Green, Blue. So, what is going on? Is the Internet wrong?
Like so many things in life, the real answer to what are the primary colors is, “It depends.” In this case, it depends on how you are using the colors. If you are feeling like an artist today and have taken out your oil paints to paint a landscape, the primary colors are defined as those basic colors which cannot be created by combining two or more other colors. In the case of paints, you cannot combine any two or more other paints to get yellow, red, or blue. Therefore, these are considered primary colors. Well almost.
This is somewhat of an oversimplification. Most of you are probably familiar with color printing. Most commercial color printers use a CMY color scale. When mixing inks such as those from your color printer, you generally have three ink colors cyan, magenta, and yellow (thus: CMY), not blue, red, and yellow. The colors when deposited on paper are subtractive. What this means is that suppose you place a drop of yellow ink from your printer on a piece of white paper. What that ink is really doing is absorbing all the other wavelengths of light except yellow which is reflected back off the paper. Now suppose you were to take a drop of each of the other two ink colors from your printer, cyan and magenta, to create a pattern like the one on the left.
Now you can see that the overlap of all three colors produces black. This demonstrates how your color printer can produce black text even when it no longer has a black ink cartridge. In addition, the overlap of any two of these ink colors produces red, blue, and green. What is really happening is that yellow removes the blue from the underlying right paper. The more intense the yellow is, the more blue that gets removed. Magenta removes green light from the underlying white paper and cyan removes red from the underlying white paper. This means that it is a subtractive process because it removes an underlying color. By varying the intensity of each of these three primary colors and the degree of overlap, you can produce all the colors that exist. Bear with me for a moment.
Light works differently. When we are talking about light either from your computer screen or your television screen, you have diodes (or in the case of older TVs, phosphors) that are red, green, and blue. It is by changing the intensity of light from each of these tiny diodes or phosphors that you perceived color. In fact, as proven by Issac Newton many years ago when he was playing with prisms, you can create white light by combining red, yellow, and blue light. Combining different frequencies of light is considered additive color mixing because they effectively make more light when they are mixed. (Think of it as the photons of light from each color cannot simply disappear and therefore must be additive.) You can prove this by taking three flashlights and covering each one with either red, green, or blue cellophane and then shining the flashlights on a wall in the dark with each beam partially overlapping the other two. You will get something like the image on the right.
Note that in the area in which all three lights overlap, the color appears white and is brighter than any of the three individual colors. What about the areas where only two of the three colors overlap? Here you can see that red and green produce yellow, red and blue produce magenta, and green and blue produce cyan.
Look at the two boxes I’ve described above. If we say that two additive colors like green and blue create cyan as shown in the second box, then does it make sense that when you take the pigment cyan and mix it with the pigment yellow that removes blue, that you get green? Similarly, if red and green light produce yellow, but a yellow pigment mixed with magenta removes the green and produces red?
Get it yet? If not, study these two figures and see how they are really related to each other.