Teach your child not to take aims with water guns at people's faces. Also, advise him to respect other's wishes, and not force an unwilling friend to play.
Even natural colour powder, when ingested can be dangerous for children. Intake of colours can cause severe outburst of reaction and food poisoning. Teach your child to avoid oral contact with colours.
Apply generous amounts of petroleum jelly or moisturisers on your child's skin. To safeguard the scalp and hair, apply coconut oil.
Make your child wear full-sleeved shirts and long pants. The idea is to prevent exposing the skin to colours as much as possible.
Synthetic colours are a big no-no. Non-toxic, organic, natural or herbal colours are better as they don't contain any chemicals. It will be softer on your child's skin, and will be easier to wash off.