Legal Question in Real Estate Law in California

I don't even know that this matter is addressed by law in California. Our statutes generally avoid legislating anything having to do with appearance and esthetics. There is certainly nothing about the appearance of fences (with the possible exception of Civil Code 841.4 declaring anything over 10 feet as presumably a spite fence and therefore a private nuisance).

If I had to guess how a court would decide, I'd say paint and appearance issues would get lumped in to the concept of maintenance in general, and that it would be the responsibility of the wall owner.

On the other hand, the owner of the property facing the wall may have a remedy. If the condition or appearance of the wall is so bad (for example, covered with offensive graffiti) that it significantly impacts the value, use, etc. of the facing property, the owner could sue for private nuisance. The wall owner would then have to choose between allowing the facing-property owner to make the repairs, doing them himself, or defending the suit.