Primary colors are generally defined as 1) the colors that can not be made by combining any other colors, and 2) can be blended to make all the other colors. Almost all of us have learned in school that there are three primary colors – red, yellow and blue. If we revisit the definition of primary colors, we will see red and blue fail on both counts. One can make red by blending yellow with magenta, and blue is made by blending magenta with cyan. (See diagram on the left.) Moreover, a wide range of colors is inaccessible if we just start with red, yellow and blue.
The true primary colors are cyan, yellow and magenta. If you look at color printer cartridges, you will see that they come in these three colors. Color theorists had figured this out by the end of the 19th century, yet most school textbooks have got it wrong even today!