If it is their fence, they likely can paint it if they want to paint it. Unless there is some sort of agreement binding the two properties, you likely do not have any power to prevent them from painting their property. They likely cannot put derogatory words or images on the side of the fence facing you.
If there is an HOA, their choice of color or design may be limited by the HOA's rules and regulations.
They cannot trespass on your property to get to their fence. However, unless the police was called at the time of the trespass, the police likely is not going to do something about it unless the neighbor trespasses again.
You likely are not getting along with the neighbor. Perhaps mediation would help the two of you in reaching some sort of compromise. Many counties and cities offer no or low cost mediation to head off neighbors' disputes from growing.
You can review the specific facts with your attorney to find out your legal options.
I agree with Mr. Nguyen and if the fence is on the line and facing you you may paint over it, however it would seem like they saved you the trouble of painting it. Do not trespass and repaint the fence. However if you are sure the fence is a boundry fence and have a survey or clear markers you can demand it be restored. Pick your battles, is this really worth a lawsuit?
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Sounds like you have a great neighbor, energetic and concerned about the neighborhood. Send them a thank you note.
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