Question is about my 7 yrs old daughter, her neck area both in front and back are pigmented very dark than her usual color, what could be the treatment?

See your doctor: There could be many reasons for this. Without seeing it and having more history on your child, determining a treatment is difficult. I would recommend you see your child's doctor.

Insulin resistance?: It's hard to say without seeing your daughter and reviewing her history, but that certainly sounds suspicious for acanthosis nigricans, a physical finding suggestive of Insulin resistance. This is very common in overweight children (and adults) and represents the pathway to developing type 2 diabetes. It would typically appear in flexoral creases, like the underarms.