My completely made up intuitive understanding of it is that it's the same as musical pitches; there are many versions of each named pitch e.g A through G that are each an "octave" away from each other. The difference between "Middle C" and the one above it is that the frequency of the higher version is exactly double that of the lower. As a listener the two notes sound extremely similar, the higher one just feels more "energetic" or something. I imagine colors would be the same if our eyes could see more than one "octave" but we are only sensitive to approximately one "octave" of light, ~ 480Thz-750Thz.

Given that, violet appears to be between red and blue in the same way the pitch of G appears to be between the pitches F and A.