Paint

Today I had a issue with a tenant in one of my rooming homes that makes me think maybe I should put EVERYTHING in writing.  Somethings seem self explanatory or common sense, but when dealing with people I think that assumption is to be determined.  I go to the house to put in the household necessities and the door to one of the rooms on the lower level is open so I glanced in and noticed the walls have been painted. Now I wouldn't have an issue with this if the tenant asked and if it was an appropriate color (neon yellow). I am upset and don't know how to handle this because for starters its not in writing, once you let one person get away with it then its like a chain reaction, dont want to lose a tenant / income, and when they leave I have to paint it back.