I was about to say "what iis 'dark'?"
Many times RCS will darken up as they age, particularly the females. I've had a few that I would have described as burgundy rather than cherry, though they were a nice cherry when they were younger. No rhyme or reason, water quality stayed the same with the same feeding schedule and water parameters. They just darkened as they aged.
As long as the population is remaining steady (and for cherries that means increasing) and you aren't seeing females drop eggs, or having a noticable die off, I wouldn't worry too much.