UMC is traditionally a mostly white denomination.
"Whiteness" or European racial background had nothing to do with doctrine, but was rather a reflection of the historical situation — Methodism is one of the oldest American denominations, since 1784, and up until it veered sharp left in the 60s, was one of the largest in the US when the free US society was vastly majority white. Methodism had been founded in England as a reform movement of the Anglican church, so obviously it was a church with "white" (European) members there, and of course family members tend to continue in the same faith.
Also, the African Methodist Episcopal (AME) church in America was founded in 1816, so it has been a long-established and well populated Methodist church for black Americans, and has long had its own hymnal and musical traditions, among other cultural adaptations. The fact that the AME has long existed would tend to discourage blacks from joining a majority-white church if they wished to worship with similar folks, even though they are free to do so since the 60s and even before. The big push to integrate the UMC came along in the early 1960s and was accepted readily in the North, West and mid-states. I do not know about all the South, but I've attended UMCs in the Carolinas that had persons of color in the congregation.