If its a pleco, thats pretty common. They tend to change their color from lighter to darker depending on their surroundings. As long as he still looks healthy, its probably ok.
Some fish change colors as they mature and others can change at any time depending on their mood or health. It's hard to say which it is in your case since you didn't say what kind of fish it is. It may be normal, but it wouldn't hurt to ask someone at the pet store where you got it.