Singapore is about as far away from a homogenious population as you can get. It likely has the most 'diverse' population in the world.
Singapore demographics: Chinese 74.2%; Malay 13.2%; Indian 9.2%
U.S. demographics: white 72.4% black 12.6%; Asian 4.8.
Break out the 16.4% of hispanics who are (in this Wiki entry) lumped in with white, and I guess the U.S. would be even more mightily 'diverse' than Singapore.
Not to mention the possibility of counting white people as German-American, Irish-American, Scandivian-American, Italian-, French-, Polish-, et al.