The current "Slot Color" panel show nothing when a bone is selected.
Maybe it could just support whatever selected as long as it can change color? The title of the panel then show Color [X] based on what is selected.

Slot, Attachment, Bone

or even changing the color at skeleton root node to key color change applied to the whole thing
Note that I just want the color panel to support bone on this feature request.
However, I saw people here from time to time asking how to change the color of the whole thing so maybe supporting keying color on the skeleton root node could make things a little easier for some people.