For me, it most definitely is not you at all. Asked a question of a similar nature a few weeks back about this as my partners ex had been into our house during our absence, and decided to do some jobs all by himself.
I apologise in advance if I cause offence, but I think your ex husband is ignorant, arrogant and quite possibly stupid. He certainly didn't engage any brain cells that day, as any normal person would easily understand in that situation where the boundaries are. I dare say though that if you've mentioned this to him, he still can't see that he's done anything wrong, and is thus making you question whether you're right in feeling livid about his and his new wife's actions.
Simple fact remains. You asked him to paint the outside of the house. He chose to go beyond what was agreed, so is clearly in the wrong, no matter what his excuses are. He may have an interest in the house sale, but first and foremost, it is your home and that should have been respected.
Hope you've explained to him how you do feel about this, and where he went wrong.