The difference between black American and white American culture in America is minimal compared to the differences between white Americans and white Polish or black Americans and black Xhosa. I mean, it obviously exists and forms the basis for some pretty strong discrimination, but you can still communicate with each other, you still have the same cultural points of reference, you still exist in the same political system, you probably watch the same or similar TV show and play the same or similar TV games. Cultural differences are significantly greater across language and customs and traditions and so on than they are across race.
Yes, there is a diversity within America's racial groups - some of those listed in the US survey as 'black' have been here for generations, some are of Afro-Caribbean origin. It's important to recognise that. But the best data we have for seeing how differerent these groups in where they come from is to see from how many different places they originally come from. On that metric, Canada is significantly more diverse than the United States. As a rule of thumb, most, if not all, white Americans were born in America. Most black Americans were born in America. Most Asian Americans were born in America. Most of them have no cultural reference other than America. This is much less true for Canada.
If your question was 'which country is more racially diverse?', the United States is quite clearly ahead. But the OP's topic referred explicitly to culture.
And yes, the United States' expansion was colonialism. I don't see how you can argue that the genocide and expulsion of the indigenous American peoples from their territories was not colonialism; I wouldn't have thought this was an especially controversial point.