Darkening of skin on stomach. Why is it a different color?

Thank you for your question and posting pictures.  It looks and sounds like you might have Post Inflammatory Hyperpigmentation (PIH) probably caused by some minor trauma.  Start by looking for prescription strength 4% Hydroquinone (HQ).  Do not buy it over the counter or off the internet.  Laws vary by state.  In Texas, physicians can prescribe and dispense in office.  We have been very successful using the PicoSure laser with the FOCUS and / or Flat Lenses eliminating hyperpigmentation with a series of treatments.  
FOCUS lens treatments have the extra benefit of reducing the underlying scar tissue.  This is one of our go to treatments for patients older than 40 and worried about the signs of aging.  This laser is safe for all skin types.  Start with sunscreen every day!  Once you have treated your hyperpigmentation to your satisfaction, you will probably need a follow treatment every 6 to 12 months depending your level of re-pigmentation.  Best of luck.

I treat a great deal of patients with pigmentation on the face and on the body.  Is it possible that it was red before it turned brown? An in person consultation is necessary to evaluate which treatment would be best for you.  In general we compound eight percent hydroquinone creams with chemical peels to accelerate the healing of this type of pigmentation.  Please consult a specialist in pigmentation for the best cosmetic results.  Best, Dr. Green