On one of my mail lists, someone conjectured instead of chlorophyll for photosynthesis, the plants on a proposed planet using bacteriorhodopsin instead. Now, I know that when halophytic bacteria utilize the rhodopsin, they turn from shades of red and purple to a yellow color, as my understanding of rhodopsin is that it changes from purple to yellow as it’s working. We wondered, if a plant were to use rhodopsin for photosynthesis (or at least metabolization), would this mean the rhodopsin would be yellow during the day, and purple at night?