How large is the tank. In larger tanks, 100 gallons or bigger, then 1.6 wpg will probably be enough. Also what kind of light T5HO or PC fluorescent, this would also make a difference. In my tank there are some that are like 4 inches where they are a little more shadowed. The majority are 2 to 3 inches but they also don't grow straight up. Even a plant that is 4 inches long only goes about two inches from the substrate.
To be perfectly honest I cant be sure that the pink color would actually come out in lower light. I have 3.6wpg, CO2 and EI dosing.