Why is my Fiddler Crab turning purple?

I bought three female fiddler from Walmart. I put them in a goldfish-like bowl big enough for the three of them with water and shells to climb out of the water with. I noticed their water was getting cold in my room, so I started using a heat lamp. Last night I noticed one has a huge purple spot on its shell, one has a little purple spot, and one has no purple. Am I burning them? Or is it something else?