Being an OLED panel, displaying white means you have to have all red/green/blue subpixels on at the same brightness, and when you have an always on mode you’d want them to be as low power as possible. Slight differences in the pixels around the panel could cause some subpixels being less/more sensitive or have slightly different viewing angles shifting the colours a bit.

Which makes me wonder, is pure red/green/blue text an option? It would cut the power use in third and look rather cool.