Paul, the primary colors for light are red, green and blue; if you shine
three flashlights - one red, one green, and one blue - at a single spot, the
spot will turn out to be white. I think that makes it additive color. As I
recall, red, blue and yellow are the primary colors for painting, which is
subtractive color (paint absorbs certain colors of light and allows others
Someone will come along with a much more cogent response, but if you
remember that R+G+B=W for light, you're pretty much there...