It is not possible for an external heat source to change the color of your eyes. There are other conditions that can cause your eyes to look darker or lighter in color. You should see an ophthalmologist to determine if there is a reason for the change in the color of your eyes.
I've never heard that the heat can change the color of eyes. Actually, the eye healthy is more important than beauty. I read an article on this woman who had brown eyes and wanted blue eyes so she had the surgery to remove part of her eyes and put blue disc things in and then she ended up blind. Beauty is pain and all that. but not that much. It's not worth it.