Melanin is the substance that is responsible for bringing color into the skin. Thus skin bleaching involves stopping the formation of melanin and also removing the existing color on the pigmented spot.
Scientists have found out that an enzyme called tyrosinase activates the production of melanin. So, to stop the formation of melanin on the discolored spot, this enzyme activity must be stopped.
Some of the ingredients that are used to stop melanin from getting produced are hydroquinone, kojic acid, and azelaic acid. But currently, the USFDA only recognizes hydroquinone as a bleaching agent.
Other ingredients in bleaching creams are termed as brighteners or lighteners. Research is being carried out on other molecules and some of them are getting incorporated in bleaching formulations.
For removing the existing excess melanin from your skin, AHAs and tretinoin are commonly used to exfoliate the skin and replace it with fresh skin. You should talk with your doctor and only then proceed for skin bleaching.
He/she will guide you on the correct formulations that will not only remove the existing color but also stop the further color from getting added. Also, you should take care that any product that irritates your skin should not be applied.