For the slider, I probably wouldn't have a icon to the left as a label. For a lot of sliders we are starting to put the text inside the slider area...so it would look like this.
[----Rotation: 8°--- ]
The other comment that might need some discussion is where to put this. If this gamut mask later gets added to another docker like the Advanced Color docker, how will the rotation be changed for that. It would be nice to have a central place to change the rotation across all dockers. Maybe the rotation slider would be better to put in the gamut mask docker. The rotation is changing the gamut mask, not the actual color picker.
Having rotation is a great idea. Great job with everything so far
Thanks for your feedback.
I will drop the icon and add the label to the slider.
As for the placement of the rotation controls, it depends on how the advanced selector turns out (M136). The idea was to have a common toolbar for both selectors that would tell the artist that the functionality is available and where to set a mask, and if a mask is set, let the user rotate it in the selector directly. But I am not opposed to something completely different :)