Purplish means a bit purple, not fully purple. The -ish is doing the work: it pulls the colour towards purple without landing on a true purple. A bruise, a sunset, a cabbage leaf, a shadow on skin — those are often purplish. It does not tell you which purple, red-purple or blue-purple, only that purple is in the mix. It is a calm, ordinary adjective. You can put it before a noun (a purplish dress) or after a linking verb (the sky looked purplish). Nobody will think you are being fancy if you use it.