> Do you say "straight, black hair" or "straight black hair"? The latter looks nicer, but I've never been sure.
It should be "straight, black hair."
It's about modiying. If both adjectives modify the noun, you need the comma. "ugly, yellow teeth." The teeth are both ugly and yellow, so the comma is needed.
"Dark brown hair" doesn't need a comma because dark modifies brown, and brown modifies hair. Even though the hair is dark, what's being said here is the brown is dark. You can also have "dark, brown hair" because the hair can be dark and brown.