I got the following answer per mail from a rdoc-developer, after i suggested implementing a :constlist:
directive, that would get COLOR_BLACK = 0
simply documented as COLOR_BLACK(0):
I'll see what I can do, but I may not be able to add it to RDoc 4.0 as my time is limited.
I think I can make this work by putting the constants in a section (internally) using a separate directive that also marks them as documented for the purposes of the coverage report.