It's now actually repotted in a GHE WaterFarm with just hydro leca instead of soil. Works well passively like this. I don't know can the pH of water change the color? It's different again. It's happy outdoors now but I'll have to watch the weather forecasts for extreme wind or cold.
Some of the flowers are now quite yellow instead of white.
I've personally only seen change in colouring due to change in temp (but never in a phal). If this is being kept outside atm, it could be greater diural temp range or perhaps overall cooler temps. Just a guess though...