To tell you the truth, I'm not sure either.
When I had my first batch of RCS they realy weren't red. I didn't cull. Didn't selectively breed. And so forth. But generation after generation they got more and more red. They were in a 10g, 36w PC, inert lame gravel, tap water, and 78-80F water (heater). They FLOURISHED! 200+ in a 10g easily. All deep deep red. Sadly I lost them all to a heater malfunction.
Now this current batch from Epic really aren't THAT red. They are in 64-68F water (unheated) and are dull. So we will see how that goes. They've yet to breed although I did introduce two of the last remaining females I have to the tank, hoping the males were old enough to do their jobs.