What Causes Low Amounts Of Eumelanin In People?

Hello. I know this sounds pretty weird, but I want to find out more about eumelanin. Considering it's pretty much determining your hair coulour, I would like to, in theory, reduce the eumelanin in my body. What substances counter-act/block emelanin? What is the real reason as to why some people develope low amounts of eumelanin?