鲜艳 is specifically about colors being vivid and eye-catching. It's always positive when describing flowers, art, or festive decorations, but can be neutral or slightly negative for clothing (implying 'too flashy').
usage
Compare with 艳丽 (yànlì, gorgeous/glamorous), 明亮 (míngliàng, bright), and 鲜明 (xiānmíng, vivid/distinct — used more broadly for contrasts and distinctions, not just colors).
