Even more context: in 1970 Gil Scott-Heron released Whitey's on the Moon which is a description of the struggles of black Americans interleaved with the chorus

Even more context: in 1970 Gil Scott-Heron released Whitey's on the Moon which is a description of the struggles of black Americans interleaved with the chorus "but whitey's on the moon" (another classic The Revolution Will Not be Televised was on the other side of the record). He was hugely influential in the early years of rap and hip-hop, so I've little doubt that Prince was at least partially inspired by him. It became a rather common expression of the contrasts in Amerca.