In the additive color model, red, green, and blue (RGB) are the primary colors, and mixing them together creates white. This is the way light blends together-- shine a red, a green, and a blue spotlight in the same place, and it will make white light. You add to get white. To get a lighter color use more of each color, or to get a darker color use less of each color. Additive is the color model used to display graphics on your computer screen, where all the colors are just combinations of the colors red, green and blue. Also called RGB space.
Subtractive Color Model
In the subtractive color model, magenta, yellow, cyan, and black are the primary colors. They are also called CMYK, with K standing for black because the letter B is already taken by RGB. Mixing cyan, yellow, and magenta together creates a dark muddy brown, so this is why black has been added as the fourth primary color, to get clean blacks. You subtract to get white. To get a lighter color use less of each color, or to get a darker color use more of each color. Subtractive is the color model used for working with pigments, as in painting and color printing.
In RT3D, the subtractive color model governs how colors are blended together, like with transparency and texture blending. Since CMYK are the primary colors, they help describe what subtractive means, but you can use any colors. In fact, since RT3D engines display on a computer screen, you are really in the end just using the additive color model. The subtractive color model can only be simulated in RT3D, to get certain effects.
See color models.