Suntanned is the adjective: it describes a person, or a part of a person, whose pale skin has gone brown from time in the sun. 'He came back suntanned and relaxed.' 'Her arms were suntanned but her back was white.' In ordinary speech people usually shorten it to tanned, which means the same thing; suntanned sounds slightly fuller and a little more written. It goes both before a noun (a suntanned face) and after be or look (you look suntanned). Note that it only makes sense for skin that was pale to begin with — you would not describe naturally dark skin as suntanned, and doing so can sound clumsy or worse.