So, can you paint your side of your neighbour’s fence? Legally? Nope. Practically? Only if they’re cool with it. Morally? Please just ask. It’s a ten-minute conversation that saves a year of awkward hallway glances.
In the end, a good neighbor is worth more than any paint job. Unless their fence is truly hideous, like that one house with the neon green slats. Then, maybe, just maybe, you can offer to buy them a nicer fence. Over coffee, of course. Cheers.