I've recently had the same problem with my panther... he's pretty dark most of the time. I thougt it was a heat issue, so I put in a stronger basking lamp. He still remains dark most of the time. He was moved to a bigger (2x2x4) cage in October, and hasn't lightened up since his move. Would it be a security thing? His new place isn't as well "grown in" as his old place was.