I have undergone laser treatment for pigmentation on my face, but shadow or discoloration and melasma appeared afterwards. Please advice any remedies to lighten or whiten the spots.

Hyperpigmentation after surgery (including laser) is a VERY common problem in Asians (and other Mongoloid races). I would advise you to try it on a small and inconspicuous spot before you embark onto all the lesions. Anyway, that might be too late now. Remedies are available but not very effective. There are some "depigmenting" creams but they must be used under the supervision of a dermatologist.