yes its true they can live in that temp. and they still take advantage of the relationship at that temp, but at over 75 degrees they start shedding zooanthellae from themselves. But yes color doesnt have alot to do with zooanthellae. As far as where i found that info , i have read it lots of places. There is only actually like a 2 degree zone where corals arent constantly expelling zooanthellae, + or - that area and its a constantly unstable envinroment for the microalgae. Luckily tho there is so much of it we would never know the difference.
He asked is it possible for zooanthellae to die and the answer is yes and its also possible for it to shed it for any number of stresses wether it be lighting changes, stinging neighbors water quality and temperature fluctuations etc etc. water.
And just because a coral is growing in knee deep water in the reef zones of the equator and the water is 90 degrees doesnt mean its not expelling zooanthellae at the time.