Scorpion colour change

My Juvenile Emperor scorpion (Has not moulted in over a year, but has a light colour Telson, not quite the dark brown of adults) 's sides has turned a dark brown instead of the usual light brown/white(ish) colour.

I noticed that this started on one area of his side about a month ago, but now it has spread to the entire exposed area between the exoskeleton, and now the entire area is a dark brown.

Does anyone know what the cause of this may be? He doesn't seem to come out of his burrow anymore (Used to every night and wander around the tank), he just sits at the entrance. When I check up on him, he doesn't move until I actually touch him or a prop in his tank does, and then he moves normally.

He's also still eating (I feed him 1-2 times a week). I'm not sure what to think.

Any help?

Could be premolt, just making a new shell underneath the old, could be the cause of a color change. or he's just dirty, or his shell is just getting dull.

Okay thanks!
I do hope that he's getting ready to moult, because his exoskeleton looks like it's getting pretty warn down.

He has been acting a little sluggish, so hopefully that's the only issue.

Thanks again =3