There is yellow turbinaria, you know. I had a piece a few years back (it was called yellow scroll) and it turned an orangey color in my tank wherever it was exposed to the MHs, but where it was shaded, it remained yellow, as did the polyps. Even though there are a lot of colors out there that don't seem possible, a lot of them are, in fact, naturally occurring. Just look at some species of acropora. You get amazing color morphs depending on the lighting and water conditions, but sometimes when you move them, they change and people start saying they're dyed. Not to say that it's definitely not dyed, but I have yet to see a turbinaria that was light colored enough to be able to be dyed yellow, unless it was completely bleached, in which case, it wouldn't be coloring up in your tank. Try and move it around your tank and see if it starts to regain the yellow coloration. I don't know if it would need more intense lighting, or less, but one or the other might return it to the original coloration.