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I propose two different ways to solve this... the first is to make a vertical version of the half block slabs that exist in the game.... this way you could have your one block wide walls with different colors or textures on either side. The second option would be to make it so that paint only covered one side of a block instead of the whole thing.
The first idea might also nrequire the addition of inside corner pieces.... basically stairs turned on their sides..... I realize using half blocks in this way could create problems for things like light switches or other wall mounted items, but I think work around could be found for those things.