bleaching of an anemone (explelling of it's symbiotic zoaxnthalae algae) may or may not be a good comparison...

It is a reduction of algae that absorb light, but are we really comparing algae saturation in coral specimens or simply different pigments? you could have pink and purple acroporas for instance that have a similar number of algae but the question would then be, does purple or pink provide better protection from more light, and or, does on need to be closer or further away because of it's pigment and it's ability to photosynthesize.