It's a combination of skin tone, ethnicity, culture/subculture, etc. Every piece of the puzzle further categorizes you, making it easier to stereotype.
I'm white, but the moment I start throwing around the y'alls, I'm a Southerner - a bit lazy, a bit stupid, polite in a folksy Southern way (sweet tea and "bless your heart"s for everyone!), incredibly racist, sleep on the floor while my prized gun collection gets the bed, and love Nascar, Dolly Parton, and Jesus in that order. If I point out I live in Kentucky, replace Nascar with horse racing and add drunk on bourbon, walks around barefoot, and probably fucked his cousin in high school to the list. The lazy and the drunk on bourbon are the only accurate ones.
Speaking of which, I love this comedy sketch about Southerners and Hipsters: https://www.youtube.com/watch?v=TbRVr0_ak48
Anyways, this is true for all races. If a black man has dreads, a billed hat turned sideways, pants that are hanging down below their waist, a pair of Air Jordans, a gold grill, and a bunch of bling, they're going to get pegged as something different than a black man who has a short hair, nice but not gaudy glasses, suit and tie with matching hat, polished black dress shoes, and the only bling being a rolex watch on the wrist.