If he’s still offering to do the job, and he purchased the paint for it, it doesn’t seem right to force him to keep the paint because of your sense of “mistrust”.
The painter can always go and return the paint, or use it by a different client of his. If he was a responsible person, he would have found some other way of getting to the client's location, but he did not show up at all - so he shows that he does not value your time. Always do your research, find online reviews, get references - do not blindly trust anybody, and do not pre-pay.
My landlord put a deposit with a painter, who was contacted to paint an entire house -- he showed up for 3 days, and then disappeared for an entire month because the painter found a better paying contract somewhere else!