Learn Russian: У него седые волосы.

In Russian, different words are used for the color grey depending on what you are describing. Седой (plural седые) specifically means grey-haired and is used almost exclusively for hair, whiskers, or beards. For everyday objects, animals, or weather, you must use серый.

Why is волосы plural when the English translation uses the singular hair?

In English, hair on a person's head is treated as a singular, uncountable mass. In Russian, it is treated as a collection of individual countable hairs. A single strand is волос, so a whole head of hair takes the plural form: волосы. Because the noun is plural, the adjective седые also takes the plural ending.

What case is седые волосы in this sentence?

They are in the nominative case. The standard Russian construction for possession (у plus the genitive case) literally translates to 'By him are...'. Therefore, the thing possessed—in this case, седые волосы—is actually the grammatical subject of the sentence, not the object.