I haven't messed much with this yet, but Roger Miller mentioned something in a forum the other day that I thought was interesting. We were talking about heavy daily Excel use and how that seems to make certain plants really red. Roger theorized that heavier C levels in relation to NO3 levels might make plants produce more red. Based on that, when I get around to trying it, I'll crank my CO2 to 40-50+, drop my NO3 levels to 10 ppm, and see what happens.
If you give it a try, please report back your findings!