You can't do much for pigmentation of the skin on  abdomen, thighs, and breast. It is a part of pregnancy. It may fade spontaneously but there is no active treatment. For your exercises, it all depends how active you were before your pregnancy. If it is normal delivery then you can start gradual exercises after 2 months. If it is Caesarean you should start your exercises after 6 months.

Nothing can be done fr this May settle 6 weeks following delivery After 45 days from the date of delivery U can strt exercise

Skin pigmentation will fade on its own but will take time. You can start exercising after 6 weeks of normal delivery.