they cant get your fish sick... no one would buy them if they did. but watch out for some. they might sting!!(mostly anemones)
i do think the colors would fade if not enough nutrition was provided.
I think what NYWATERS is talking about is the fake coral. Yes I have seen some very life-like vivid man made coral. I have not ever heard of the color fading and hurting your fish though. I wouldn't worry about it.
