Use Dimensional fabric paint, applied with a cocktail stick. You'll find the suff in any Walmart or craft store. It's normally used to decorate t-shirts and the like. Main makes are Plaid and Tulip, but others abound too... They come in lots of different colours so you can chose a color that will stand out over your plastic (saves your eyes!). It's resistant but to be on the safe side, seal with a light coat of Future (with a brush) then paint, decal, weather as normal. Best stuff around for rivets, I assure you!