My understanding is that the brown colour is caused by an increase in the amount of zooxanthellae within the coral tissue as it was not getting enough nutrition from those it had, probably due to a reduction in light.
Over time, with better water quality and light the coral will expel the excess zooxanthellae and the original colour will return. Keep in mind though that any coral will potentially take on a different colour in different tanks due to differing environmental conditions.
This is an example of what I am talking about - I have no doubt the the current colour of this coral is different to what it was originally but it is definitely a lot better than it was.
I agree with Matt - it is hard enough to keep SPS corals without taking on one that has obviously been stressed. Depends on the price of course;)