绀 is a bound form (b.f.) meaning 'dark purple' or 'deep purple with a bluish tint'. It rarely appears alone in modern Chinese; you'll mainly encounter it in classical or literary contexts, or as part of compound words like 绀青 (gànqīng, 'dark blue') or 绀紫 (gànzǐ, 'dark purple'). As a bound form, it must attach to another morpheme to form a word — you cannot use it as a standalone adjective in everyday speech.