I was doing a search on red plants, and I think Sarah might be on to something. For a long time, my 10gal had only 36W @ 10000K and the ludwigia was consistently a nice dark red. I was having a small problems with hair and thread algae which I ignorantly blamed on the color temp, not the fact my fert dosing sucked. So out came the 10000K bulb (it had cracked accidently) and currently under the hood sits 1x36W @ 6700K and 2x13W @ 6700K. More light, less red. The underside of the leaves are a beautiful shade of red, but under the 10000K bulbs the tops of the leaves were less green/more red as well. The Rotala indica is only slightly pink at the tips. Like Anthony, I dose macros on S days and T days, .5ml-1ml traces and .5ml Flourish Iron on MWFs.
I may order a 10000K bulb to carry out a three week ultra-unscientific experiment.
....hmmm I just noticed that I said my dosing sucked during the 10000K period. Could it have been the lack of nitrates that were the source of the redness as well as the hair algae? If plants exhibit more red at low nitrate levels, how low is low? 10? 5? 2? 0???