They can, though they appear more muted. Surely a dog can tell the difference between a black and white person, though.

I thought a little about what dukedogsmom said. I think if you're a white person and you live somewhere where Black or Hispanic people truly are in the minority, a dog just might bark at them because they're not used to them. One town I lived in in Texas had a 55% Black population, and a good-sized Hispanic population--Lefty probably would never bark then!