I studied all this once... let's see if I remember it right....
There are two methods to combine colors: additive and subtractive.
Additive is the method used when one mixes pigment. Ths is the one most people are familiar with, where Red, Yellow, and Blue are the primary colors that all other colors are derived from. People have been learning this since they were old enough to color or fingerpaint. In a print shop, this is the CMYK model.
Subtractive is the method for mixing light. This is where the RGB model comes in, where red and green make yellow. Most people aren't as familiar with this model. Until the last few decades, most people never mixed colors in this way, and it's therefore much harder to know intuitively wich colors to combine to create a new color.
For example... everyone knows that red and yellow make orange using the additive method. What makes orange in the subtractive method?
Aaron
http://www.voodoovibe.com
p.s. White is the absence of color using the additive method, but the sum of all colors in the subtractive. Vice-versa w/ black.