I've found can's of paint around the wasteland. There is even a mini quest connected to Diamond City concerning paint. But can you custom paint your power armor? Give it Camo color or anything like that. I know there are a couple of options when upgrading the armor, one for charisma for sure that say that your painting the armor. But you don't really paint it.
If nothing like that is in game yet, it sure as hell would be a cool mod to put into the game, or even some cheap DLC that lets us put custom paint jobs on the armor. I'd fork out $5 USD for that.