When painting a car wash, don’t go cheap and stick with the big name paint stores (Benjamin Moore, Sherwin Williams, etc.) and look to a latex or oil based epoxy. Talk with the local with the rep to get the blend and colors right. This isn’t an area to try to go cheap as the cheap paint doesn’t stick well. Expect to pay $300 for 5 gallons.
If you can get away with it keep your painting at a minimum or you will be freshening it every year. The curse of a car wash is paint–any paint. Try tile, plastics, metal, stained block or other coverings. Keep paint to an absolute minimum and you will love yourself for many, many, years down the road.