This command rotates the hues of both new and existing rings by 90°. The background color is not affected. The rotation is cumulative, so after four repetitions you're back where you started. Only hues are affected, not lightness or saturation. The effect is more dramatic when hue cycling is limited; see Loop Hue.
Hues can be changed gradually via MIDI, or by moving the mouse vertically while holding down the right button.