I mean... also, if someone popped out of the door to take a swipe at Valmont, he would've noticed the door opening, right? He wasn't totally oblivious, and chances are he was on edge since he... was, er... probably gearing up to murder Dylan, judging by the knife and the letter...

It's more likely that someone attacked from behind, or any number of other hypotheticals. We can't say for sure that it was anybody whose door was painted red.