my clownfish is turning black??

I have a 50g reef. I have 2 clowns in there one black and one orange plus some other fish..people have said not to put them together but I have no problem with them not getting along. Both of them have hosted this big polyp rock I have in my tank but my orange is now starting to turn black? Not completly but he has black spots and it looks like his actual skin changing color not anything on the outside what I'm wondering is if he is just naturaly changing color or is it a sickness? Or maybe arethe polyps stinging for some reason? But they are both acting normal they just kinda hang out in the polyps and go crazy at feeding time... any body know what's going on with my litte guy?

 

My guess is that it's a natural change of color. I have a piccasso clown that has begun to turn black by his dorsal fin. From my understanding, it is a natural part of their aging.

 

If its a color change then it is an enviornmental adaptation thing... kinda like a chemeleon

 

It's natural. My clowns began to get the black spots as they aged. I also noticed the spots began to increase in number and size when they began to use stinging corals as hosts.