When my son’s SWCAH was not well controlled due to the fact he cleared his meds so quickly and did not absorb them properly, he became very thin but developed a tan, although he did not go out in the sun, (he has a fair skin).  His knuckles and the creases in the palms of his hands, ankles were darker, almost if they looked grubby.  He then developed dark brown patches on his hands, elbows, stomach, he became yellow around the mouth, and the soles of his feet were yellowish.  When he started on the pump, after a couple of months all this disappeared, I too was told it was from the high ACTH levels.  I would certainly ask the endo to check ACTH levels, try to get an early morning 17 OHP level and an androstenedione level.  Good luck.