* Hair as a singular, uncountable noun: In English, we often treat "hair" as a singular, uncountable noun when we're talking about the hair on someone's head as a whole. Think of it like this: you're not counting individual strands, you're referring to the overall mass of hair.
* Example: "Her hair is long and brown."
* Plural "hairs" The plural form "hairs" is typically used to refer to individual strands. For example, "I found a few hairs on my coat."
In the question "What color is his hair?" you're asking about the overall color of the hair on his head, not counting individual strands. Therefore, it is treated as a singular, uncountable noun.