Perhaps it was stressed in the store and is happy now. Or perhaps it was some sort of mating display or show of dominance and there is no longer a point in displaying the colors being alone. Lighting might alter it's color slightly, but not dramatically, so I wouldn't advice trying that.
The first thing to do is to find out what the fish is. <A HREF="">fish base</A> does not come up with a orange spotted rabbitfish. Dr. Mac's website no longer lists them, if it ever did. Perhaps you could e-mail him and see if you could find out there scientific name or at least the genus.
.............
Well one last thing to try... google. The first result under a search on google results a page you may want to read. <A HREF="">Does this look like your fish? </A> If so we come to reason number 320 you should try to ID your animals as much as you can. <B><U>"THIS FISH IS VENEMOUS. The dorsal fins of this fish can cause extreme pain. Severe reactions can occur if allergic. If stung soak injured area in hot water and seek medical attention. "</U></B>
If that is not the exact fish you have or you are not sure, start looking at that genus, Siganus, or under the second common name listed, Gold Spotted Foxface.
Good luck, keep us updated,
Jon