Here are the two sun corals I purchased back in October. One of them was purchased as a "red" sun coral. It was from Aquatic Collection and looked pretty much identical to their photo at the time I got it.
Boring speculation on "red" sun corals...
It had a bright pink/red coenosarc and brilliant reddish orange polyps. It has faded gradually to what you see today. I have my doubts about any tubastrea being dyed. Typically, it's done to zoxanthellate corals that are bleached (forced to expell their symbiotic algae) and become pale or white in color. Tubastrea's color is not based on zoxanthellae--they don't have any--and I have never heard of them bleaching. So I'm not sure how one would go about dyeing a coral that is already so intensely colored.
I'm wondering if in "red" sun corals that fade over time, the culprit is diet. I expect they get a much more diverse diet in the wild, probably with a lot of crustaceans that have been eating phytoplankton. Phytoplankton are high in carotenoids, a set of pigments made by plants but responsible for bright colors in animals.
Carotenoids are responsible for red, yellow, orange, pink, and brown colors, so I wonder if diet isn't the reason a lot of sun corals change color after a few months in captivity. They exhaust what's stored in their tissues and when they don't get any more, the colors change. Since we feed meaty items, they probably aren't getting the same amount of carotenoids they get in the wild. This is one reason that the colors in some marine fish (firefish, some dottybacks, others) will fade if they don't get a good mixed diet in captivity. Maybe the same thing is happening in tubastrea.
That's my guess about what's going on with red sun coral, anyway. I'm going to hit the fish store and buy some meaty foods that are rich in carotenoids and start feeding them to see what happens. So far I've been feeding them mysis and I just added some krill to their diet. Krill sometimes have some carotenoids in them, but not nearly as much as some other foods I think. Cyclopeeze is supposed to be really high in some of the carotenoids, and there are several specially made foods to color enhance fish that would probably work. I'll see what I can find.
What specific items do you all feed your sun corals? If you have had them for a while, have you noticed any shift in color?
End rant...
Anyway, other than the color change, it looks healthy

It's grown two baby polyps since I got it and recovered from a lot of tissue recession.
And here's the black one, which is possibly
T. diaphana. I also got it as four polyps and it's grown five new ones in two months! Wow!
I love my sun corals. I get a huge kick out of feeding them.