IanDB said:I don't see any reason why you can't paint your side of the fence a different colour though. As it has now become a party wall (and that then starts another wonderfully vague legalistic route) you can do anything you like to your side.
Painting your neighbour's fence:
TV gardening programmes may inspire you to stain or paint the fences around your garden in some colour other than creosote brown. The chances are that some of these fences belong to your neighbour and not to you. Strictly speaking, you must ask your neighbour's permission to paint or stain your side of his fence - it is after all his property and on his land. What if he says "No"?
If he says "No!" then you must leave his fence alone. If you go ahead and stain it anyway then you are doing criminal damage to his property.
This is a clip from This site