You don't understand how white people contribute to hip hop because you don't understand the Beastie Boys, the group that laid the blueprint for how white people contribute to hip hop. It's not colonization, it's more like translation. The Beastie Boys showed how all of the conventions of hip hop work when applied to white culture. The Beastie Boys contributed to hip hop precisely because their music contributed to white punk culture. Because rap is about being true to yourself while still representing you environment. If you feel like a punk inside, you can make hip hop music as long as its still punk music. If your cultural mileau is white punk culture, then the hip hop thing to do is to represent punk culture, which the Beastie Boys did and in doing so influenced all future white contributors to hip hop.