There have been many threads about this. A lot of vendors (and hobbysists) will take pictures in a RAW format and then post process in Photoshop (or equivalent) to white balance the photos. There are times that is taken a bit far which alters the look of a corals. There are vendors that take very realistic pics of their corals and others that go overboard (and I'm not referring to any sponsor here). The best thing to do is do your research on vendors you want to buy from. Check their feedback here and on the internet. If you like what you see, place an order.
Also keep in mind that many vendors take pictures "top-down" which will likely look different from a "through the glass" view. All of these things are important to keep in mind. There are a lot of very reputable vendors out there. I personally buy from a few of them. You will find your "go-to" guys who come through with killer pieces that are well represented in photos. Hope that helps!