'slate-colored' originates from English, specifically the words 'slate' and 'colored', where 'slate' referred to the fine-grained metamorphic rock used as a standard for a particular gray-blue hue and 'colored' is the past-participle adjective formed from 'color'.

'slate' comes from Old English 'slǣt' (or Middle English 'slete') referring to the rock; 'color' comes from Latin 'color' transmitted through Old French (cf. 'colorer'/'colour') and Middle English; the compound 'slate-colored' developed in English by combining the noun 'slate' with the participial adjective 'colored' to describe that specific hue.

Initially it meant 'having the color of slate' and over time that basic meaning has been retained; later it also acquired occasional figurative senses (e.g., 'somber' or 'muted').