I've found the best return on light per time is to balance your production so every color fills up at as close to the same rate as possible (as in all your tanks get full at the same time). Then you prestige once all the tanks are full.
That does lead to a longer time between prestiges but the returns per time are significantly higher than just rushing white or any other more micro intensive strategy I've messed with. It's a little unintuitive to me, so maybe this will help people. Alternatively it's more obvious than I thought and everyone already knew that.