Scuff it up on the outside a bit with a sanding pad then spray paint it the color you want (a couple of thin coats should do), then adhere your design on the top with quick-drying glue.
Although I don't use mine for "art", but for storing my embellishments instead of purchasing those expensive tins. I have a larger square tin (Mary Engelbreit) with a hinged lid that stores 14 of the larger altoid tins quite nicely!
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