How do we observe light? We can only see colors that enter our eye. This means that when you look at an illuminated surface you are realling seeing the light that is being reflected off the object.

Pigments or dyes on objects work by subtracting different colors of light. For example, when you look at an apple it appears red because it absorbs all colors of light except for red.

This means that if you add more pigments together more colors get subtracted out. What would happen if you added all different colors of pigments together?

As shown by the example, the three primary colors of paint or pigments mix to make the three secondary colors in these ways when mixed in equal amounts.
Cyan+ Magenta = Blue
Cyan + Yellow = Green
Yellow + Magenta = Red

All primary colors added together make black. This happens because all colors are subtracted out.