Hi,
Some amount of hyperpigmentation in the inner parts such you mentioned, is normal. This area also is rich in sweat glands to keep our body cool. Friction is caused less by the body and more by the kind of clothing we wear. Therefore use cotton clothes to help you stay cool. You may also use anti-perspirants to help you with decreasing the perspiration.
There are many treatment options available for reducing the pigmentation and a customization to your needs and requirements will be required by a doctor. Please visit for further diagnosis and treatment.
Do let me know if you have any other queries and the status of this condition.
Best regards
markie 18 man @ hot mail . com no spaces or underscores or nothing