I have found that the more light the redder the leaves and the leaves that do not get that much light stay greenish.
This is ludwiga repens (think that is what it is called) in 29 gallon high with two full length 6500k flourescent bulbs.
It grew like weeds with no co2 just 2 gallon weekly water change.
There is some jave fern here and there.
very small leaves, definitely a sign of no co2.. ludwigia repens leaves are about the size of a quarter when receiving all required nutrients