I have also observed this in Bacopa caroliniana in a tank with 55g, DIY CO2, and 1.75 wpg with a 96w PC from AH Supply. The plant would be deep pink in the mornings and turn lemon green in the afternoons right before the lights went off.
I tested for PO4 and NO3. I found my PO4 to be practically unmeasurable. My NO3 was 5 ppm.
I started dosing small amounts of PO4 in the mornings. I noticed that a few hours later, the Bacopa began to turn lemon green earlier in the day. I added more to the point of having 0.5 ppm. The Bacopa was no longer pink in the mornings (nor have such short internodes).
Just my experiences,
Carlos