Could be a million reasons. Need way more information. Like how long has the tank been setup. how long you have had the corals, what are the corals, , what is the temperature, what are the nitrates (0 nitrates can cause loss of colour), what else is in the tank ( corals) what are their colours like etc.
Often it is just the stress of cutting a frag and moving to a completely new tank will cause a coral to go brown. The good news is fix the problem and the colour will come back. If the problem is just stress sometimes it takes 6 months to a year for a coral to recover. Higher end corals are more temperamental which is why they are higher priced.
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