Ask at several paint stores -- not Lowes/Home Depot.

Based on the rock texture .... SPRAY paint!

Driveway paint, which has an acrylic base, would be a good bet as it is made for concrete.

Exterior latex, maybe satin or gloss to resist water a bit rather than flat. Higher quality may last longer. How long has it been since last painted? If it lasted 20 years at this point I wouldn't go overboard over thinking it. It will be better than it was. Maybe you can get some of the local stores to donate paint, then take whatever you can get in white. Different shades of white won't be noticable at a distance. But if you mix several gallons together in 5 gallon buckets it will be even less noticable.