Spray the paintball paint with cold water from a hose.
Scrub the paint with a brush. Spray it again with water.
Mix 1 gallon of water with 1/3 cup of TSP if there is still paint stuck on the siding. Scrub the siding with a brush or sponge dunked in the solution. Scrub the siding a second time if it doesn't come off the first time.
Add another 1/3 of a cup of TSP to the water If the paint ball paint is still sticking. Add 1/3 of a cup of laundry detergent. Wash the siding with the mixture if there is still paint stuck to it.
Spray water on the siding with a hose to rinse it off.