The best paint to use is spray paint. You will probably need 2-3 cans for a good base coat and additional paint for your design. You will probably need to spray it a few times for complete coverage.
The quantity needed will depend upon the color you are using, the color already painted on the rock, and weather conditions.
Latex paint or paint that is rolled on will take longer to dry.
Premium spray paint works best (Rustoleum, etc) vs. a generic store brand.
Once you have the base coat done, use chalk to sketch out your design. Try to do large shapes so that it’s easy to see from a distance and so that you can use spray paint for most of it.
Stand back 15-20 yards from the rock to make sure you can see the design from there.
Spray paint your large areas. If you mess up, use your base coat color to fix the mistake.
For more detailed areas, use a can/tube of paint and a paintbrush. While outdoor latex is best, regular artists acrylic seems to work well too (even in rain).
If it’s raining you may want to bring a pop up tent to keep yourself and the rock dry.
Make sure to bring paper towels to wipe any drips.
Bring a cup of water if you are using a paintbrush.
Wearing rubber gloves will help keep your hands clean.