The image color normally cycles around the entire hue wheel, at the speed and direction dictated by the Color Speed parameter. In Loop Hue mode, cycling is limited to a subset of the hue wheel; hue loop length is the size of the subset, in degrees. Loop Hue must be enabled for hue loop length to have any effect.
If the loop length is zero, Loop Hue makes the image monochromatic: all rings will have the same hue, though they may differ in lightness or saturation. With a small length (e.g. 30°), Loop Hue makes the image nearly monochromatic, but still allows rings to be distinguished from one another by hue.
Hue loop length can be changed via the Master dialog, or MIDI.