In our world, there are two basic forms of color: tangible colors such as those used in paint, ink and dye, and the digital colors on a computer monitor, television, smart phone or movie screen. As you look at the diagrams below, notice that the primary colors in each are different.
Digital Color (Additive System Based on Primary Colors of Red, Green and Blue)Digital color is pure light that doesn't exist apart from the screen that displays it. Theoretically, the human eye can see sixteen million digital colors which are created by an additive process that begins with black, or the absence of color. To create color, red, green and blue are added digitally in layers of pixels. Notice in the center of the additive image that the secondary colors are cyan, magenta and yellow, which are the primary colors in the tangible color system. Secondary colors are called that because they're made by the layering of two primary colors. The image shows how layering pixels of blue and red creates magenta. As more layers of pixels are added, the result gets lighter, finally ending with white, which you see in the center of the circle where all the colors overlap. White light absorbs no color and reflects all color equally.
Tangible Color (Subtractive System Based on Primary Colors of Cyan, Magenta and Yellow)Tangible color is a subtractive system that begins with white. The tangible system presents color to our eyes by subtracting and absorbing wave lengths as pigments are added. Sounds counter-intuitive, but stay with me. The key to understanding subtractive color is to think of pigment as something that subtracts or removes color by causing the absorption of light. As more pigment is added, the result gets darker and eventually turns black. Cyan, magenta and yellow are primary pigments that remove only one color. Adding cyan removes red, adding yellow removes blue and adding magenta removes green. Aha! Now notice that the secondary colors in the circle are red, green and blue, the primary colors in the additive system, and that when all the colors are added together the result is the black you see in the center of the circle when all colors are absorbed and none is reflected.
When colors are mixed, their light-absorbing behavior is combined. For example, if you add blue paint to yellow paint, the result would be neither yellow nor blue because the blue paint would absorb the yellow light and the yellow paint would absorb the blue light. Only green light isn't absorbed by both paints, so the result appears to be green.
Why You Should Understand These Systems
Many people have problems with color, especially tangible color in paint and printing projects because the results differ from the sample they're trying to match. It's not surprising when you think about it, because although it's possible to create millions of digital colors, because of the nature of dyes, pigments and other materials, we're able to create only a fraction of that number in tangible colors. You'll most need to understand these systems when you're trying to get a digital color reproduced exactly in a tangible one. Now you know that the reason you might have a problem is that you're trying to cross platforms, so to speak, and it just may not happen...
If you'd like to learn more about these concepts, check out this video from Youtube.