Click the control or press R to activate.
The Rainbow Intensity control, part of the Secondary Diffraction Spikes control group, sets the saturation of the color in the multicolored repeating "rainbow" pattern in the Secondary Spikes. This is mixed with the color of the star.
Higher Rainbow Intensity values produce a stronger rainbow effect.
Note that with this control turned all the way down only the star color will remain in the Secondary Spikes.