As any artist can tell you, the primary colors are Red-Yellow-Blue.  Red and Yellow combine to create Orange, Yellow and Blue create Green, and Blue and Red create Purple.  To account for all six colors of the rainbow. 

The question is:  yellow and green are both at the center of the rainbow, why is it that we use RedGreenBlue?  It can't be because of an additive or subtractive light issue.  

''It can be, and it is.  Pigments (artists' paints) are subtractive, lights (most display pixels) are additive.  See http://en.wikipedia.org/wiki/Additive_color and http://en.wikipedia.org/wiki/Subtractive_color''