What are these buttons supposed to do? They seem to have no effect at all on anything and I don't see it in the manual. It's in the Custom Gradient controls, Matte Color controls, and anywhere you pick a color, but near as I can tell, it's a dummy button!
It is called the "Hue Direction Control". If you’re keyframing changes in color, click this control to indicate the direction on the color wheel Final Cut Pro uses to interpolate the color change. Hope this helps.