We used orange chalk at the dairy, it was way cheaper and faster to apply then the patches. Several years ago I saw a neighbor use the Target or Wal-mart spray cans. At 99 cents a can, its fast, easy and you can paint several cows per can. I put it on thicker than most.
Starting a few years back I started buying 5 gal buckets of the cheapest, chalky white, latex indoor house paint and put it on with small scrub brush, it stays pretty well unless the cows start shedding bad.
I keep all the butts painted and repaint them after insemination, that way when I ride the pastures I can just watch the cows W/O paint.@