I have not done a controlled study to say that high PO4 definately gives you more red in plants, but the association of high PO4 and disappearance of green spot have been reported by many people and recommended by Tom Barr. The newer leaves will not grow green spots. The green spots on older leaves will eventually go away as well, but it takes time, maybe weeks or longer. Anyway, it is refreshing to see all my anubias are free of any algae for a change, it has been that away for a few months now. Before I dosed higher PO4, I can only keep 2-3 good leaves on any anubias at any time.
About red Ludwigia(L repens) in particular, it is not a "true" red plant if I remember correctly, in the sense that it might be red only when nutrient stressed(low NO3 and PO4). I just put some in my nutrient rich tank, will see if it can keep the red color. If it turns green, I am not going to sweat it, I have plenty of "true" red plants, that is, red in nutrient rich water(for example, rotala macrandra, rotala wallichii, ludwigia ovalis, ludwigia glandulosa, ... ). R. indica/rotundifolia only gets pink when it gets close to light, redder with lean nutrients.
Flourite, wet/dry, AHSupply 302w PC, PinPoint controller
15w T6+15w T8
40w T6 ODNO4x, no CO2. Discus grow out.
10gallon Tonina tank, 2x15w T6 ODNO 4x
I will do aquascapes after I collect all the plant species and grow them to perfection.