Hi.
What do you want to paint? If it is a pattern or shape like graffiti then you could use a decal. Ray trace from user to target … at interception paint a decal.
If you want to change the color of the walls, then your trick here would be to ensure your materials are set up properly from the start. Have the material have a dynamic colour and then do some BluePrint trickery to update the target object material at interception.
I am sure there are tons of way to do this … but this was just quickly off the top of my head.
Good luck … HTH. 8-}