Cosmelan is designed to help blend areas of Hyperpigmentation. Sun exposure will darken any skin and promote increased color, both overall and as a catalyst for unwanted concentrations of pigment. As a long term program for actually lightening skin, you may seem some mild effects, but you won't be able to lift color substantially. In other words, don't look for a bleach effect.
By blending the hyper pigmented areas, you'll feel like your skin is lighter overall. Good skin care will provide more radiance and light reflection by preventing cell buildup which can make skin look ashy or darker.
Aggressively treating African skin can end up causing loss of natural pigmentation in an uneven way, so don't undertake a self-concocted lightening program.
Sun protection is your best friend when it comes to pigmentation control.