About Colour/Three Primary Colour Theory page 11 of 15

We now have six basic colors we can group into a pair of Triads. The first triad of Red, Green, and Blue (RGB) can be mixed to produce all three colors of the second triad, consisting of Cyan, Magenta, and Yellow (CMY). Each color in the RGB triad is percieved by the stimulus of a single RGB type of light cone in our eye, and all three colors of the triad can be directly observed in the dispersal of White light.

The colors of the CMY triad can be combined to produce all three colors of the RGB triad by filtering White light. Each color of the CMY triad is percieved by the stimulus of a pair of RGB type light cones in our eye, and only two of them can be directly observed in the dispersal of White light. The very notion of "Primary" requires one of the triads to be more fundamental than the other.