Describe the issue

In #9391, I proposed to open a new issue for "colorbar should refuse to let itself be created for an RGB(A) image" because colorbars are not meaningful for images with existing color. #4369 (comment) similarly notes that this makes no sense.

(better late than never I guess!)

Proposed fix

I think that attempting to add a colorbar for an RGB or RGBA image should be an explicit error, following a period of deprecation warnings.