I used Walmart house brand flat white spray paint.
After it dried, I used one of these...
some of this...
and one of these...
Scrub through the white paint in areas where you'd expect the paint to be worn, ie the top of body lines and edges. After that I even scuffed it up with a piece of chain and my boot soles. It matched pretty well when finished. You just have to stop and back away every so often to check progress from a distance to make it match the rest of the truck.