Following on from this earlier post, from the commit-log:
Colors used by Bone Groups are now copied/assigned to Action Groups too when
they're created now. This completes the work started in r.46960 to restore this
functionality from 2.48.
Currently, there is no control over when/whether these colors are copied over
(although it is possible to disable the display of these colors for relevant
animation editors if desired). Originally I was going to make this a more
generic Keying Sets feature, though that turned out to be a bit too complex to
manage. This simpler implementation will do for now (and it's still always
possible to use scripting to tweak things to your liking/needs).