:iagree:you want red in your ludwigia?
iron, iron, iron.
get yourself a bottle of Flourish Iron supplement, and dose as directed.
In most plants, certain nutrient additions or deprevations are the reason for change in color.
For example, in L. Aromatica one needs to deprive the plant of nitrates in order to bring out the red colors. Rotala Macrandra requires an excess of iron for it's red to become prominent. Only a few plants are dependent on light for color. Quite the opposite, more light leads plants to development more chlorophyl, enhancing green pigments.
For ludwigia, an excess of iron will help bring out a red hue.