>> How do you keep young children who have access to crayons and markers from decorating your house?<<
Keep the coloring supplies where he can’t reach them so that you need to give them to him when he wants them, and supervise him carefully when he uses them. Immediately intercept him and matter of factly remove whatever he’s using as soon as you see him about to use the crayons or markers inappropriately. He’ll realize very quickly without you saying much that if he wants to use them, they will have to be used in the right way.
Isn’t it nice to have an issue that’s so easy to resolve?