The meaning "characterized by cheerful optimism" is perhaps on the notion of something uncommonly beautiful. In early use it often was applied to the mist or light that superficially brightened circumstances, and perhaps the earliest appearance of it in English is in the figurative phrase rose-colored spectacles, attested by 1830. The noun phrase rose-color meaning "a pleasant outlook" is said to be from or based on French couleur de rose (itself used in English poetry by 1831). Rosy is by 1775 in the secondary sense of "cheerful." There is a nonce use of rose-colourist for a cheerful optimist from 1852.