Additive primary colors are the primary color elements that make up white light. These colors are called additives because you must add the colors together to create white. The additive primary colors are red, green, and blue (commonly called RGB) as they are the primary color elements. Additive primary colors are the primary colors of pigment that different systems apply to the colors that we see on computer screens and the colors that we see in printed media and photographs.
Recommended Reading: Webopedia’s The Science of Color and the Digital Camera Quick Reference.