The interesting thing is that (from my personal observation) the illusion of color is best under monochromatic light. If you can get a low pressure sodium vapor light, try it.
If the rotation of the disk is reversed, the colors swap -- red to blue, and vice-versa.
That would be consistent with the geometric factor I mentioned. You would be reversing the phase of the activation of the photoreceptors involved, hence different, evem opposite colors would be expected.
That answered that!