I was taking a acne treatment. i had been prescribed lot of pills (isotroin-20), serums (vc-15). my skin goes dark in sun if i do not cover my face.

What's your question: You're on Isotretinoin which is the strongest acne medication and will prevent further scarring. The vitamin topical application probably won't hurt. You'll do well to use a heavy sunscreen especially while you're on isotretinoin. Please do not try to bleach your skin with hydroquinone; you'll end up disfiguring yourself. Don't worry about your skin being dark; it's fine.