My sulcata has always been super clean and good color. But ever i got him/her Eco earth coco substrate, his head is always brownish black and i cant get it off.
Before Eco earth coco
After
My Sulcata Michelangelo[TURTLE]
and my RedFoot Raphael [TURTLE]
I have never noticed this with my russians, but that doesn't mean it didn't happen... However I don't think you have anything to worry about, others will chime in with their opinion.
Have you tried washing it off with a very soft bristle tooth brush or something like that?
Same thing happen with my torts, skin on the head and neck area. Its either a natural colorant or they may be adding artificial colorant to coco coir and coco husk chips. When I drained the water after soaking coco coir chips it left brown stain in the driveway. As the skin sheds so does the brown color. No adverse effects seen.