This may be a term that is pretty well understood. Think back to elementary school art classes, when you would make a color wheel? Remember? You have, red, red-orange, orange, orange-yellow, yellow, yellow- green, green, green-blue, blue, blue-purple, and purple on a wheel. Red-Yellow-Blue are the primary colors. These colors mix together to create the rest of the color wheel. Pretty simple right? Red + Yellow, make orange. Yellow + Blue, make green. Blue + Red, makes purple.