What's the difference in colour between blond and fair, referring to hair?

The difference is that blond, is commonly used to refer to hair that is yellow, while fair is not ever used to refer to hair, at least not where I live. If you wanted to say someone had yellow hair you would say "blond" and if you wanted to say they had yellowish brownish hair you would say "dirty blond". If you wanted to say someone had light-colored hair, you would say just that: "They have light-colored hair". Fair is more commonly used for skin, to mean light-colored or white. "They have fair skin".