Pepper-and-salt describes hair (or a beard) that is starting to go grey: dark hairs and pale hairs mixed, like black pepper shaken with white salt. It is not fully white and not still all dark; it is that in-between speckled look, often first at the temples. People use it as a kind, almost stylish way to name ageing hair — more picture than medical fact. You will hear it from writers and from people describing a face; it is a little old-fashioned and descriptive, not slang. The same picture is sometimes used for cloth, a tweed with tiny dark and light flecks, but the sense you will meet most is hair. It says ‘mixed,’ not ‘finished turning grey.’