the three primary colors are in the corners. The color you get by mixing two primaries is between them (red and yellow make orange; red and blue make purple; yellow and blue make green). The complementary color of a primary color is the color opposite it (green is the complementary of red, orange for blue, and purple for yellow).

Almost all visible colors can be obtained by the additive color mixing of three colors that are in widely spaced regions of the visible spectrum

If the three colors of light can be mixed to produce white, they are called primary

Two colors that produce white when added together are called complementary