Just stumbled upon this site called futurederm.com and found this very interesting article...
Most important paragraphs shown below:
"Whether you’re using hydroquinone, retinoids, niacinamide, L-ascorbic acid, etc., those compounds mostly inhibit FUTURE and ONGOING hyperpigmentation, rather than what’s present NOW.
The only notable topical ingredient that can diminish existing hyperpigmentation is glycolic acid, and to an extent salicylic acid. These hydroxy acids (primarily the former) induce chemoexfoliation of the entire surface of the skin. By increasing “cellular turnover” by stimulating the skin to shed the outer layer of cells, glycolic acid quickens the eventual shedding of the excess melanin that’s sitting at the bottom of the epidermis. However, there is one caveat to this ingredient: in order to induce this type of controlled damage; at-home glycolic acid products (5-20%) do not possess the strength to significantly reduce hyperpigmentation."