I believe it is actually quite important that colors will not form a planar circle if they are captured as wavelengths or frequencies. This is easy to understand when captured in a vertical structure. It might be more correct to perceive that by the moment purple is reached, we have risen to a different layer. In that way, it will become more of a three-dimensional color helix structure than a planar color circle. I suspect that Johann Wolfgang von Goethe might have had this in mind, as purple and red have a more distinguished boarder relative to other adjacent colors in his color circle.