Stable Carbon supply to the plant is part of the issue also.
Non CO2 planted tanks seem to manage to get very nice reds as well, once established.
Light will certainly drive the rates of growth faster, this means less time for Chl a to be formed in the new tips, also, as the tip growth gets closer to the lights, it has more intensity/higher PAR, the plant used the red color to protect from herbivores, UV light(not an issue for us however) and are among the first pigments in new tissue.
With slow rates of growth, we still have good color however.
It's hard to say why really.
Stable conditions, healthy general growth has always seemed best.
Good CO2 if you use it, good light etc.
I have some really deep coloration for this plant at 1.8w/gal, good CO2, ADA AS, EI dosing etc.
I've spent a lot of time fiddling with trying to keep and maintain a very low NO3 residual. Some found this a good way to induce it for a show, etc, but few seem to keep doing it say for "years",. most forget about it after a few bottoming out of the NO3 events.
They learn and stop.
Another way is using soil or ADA AS etc, and not add much light and not add much water column ferts. Plants still have plenty of N however. Over time, the reds will pop out as the N levels go down, but this is also when BGA appears also.
There's always a trade off.
There are many species of nice red plants to pick and chose from so I just focus on that and good general growth for all plants, not just red ones.
More Red seems to be a stress, not something that suggest healthier plants.
Regards,
Tom Barr