Floors and walls are separate entities however segments are not. You can paint the separate parts of a segment by using the F key for floor and the M key for wall only modes, but this will normally overwrite the desired floor or wall anyway. You can get around this by painting your walls from the outside instead of from the inside, but this does not often have the desired effect and can ruin your nearby existing segments.
The best way to do this is to create your own walls and floors using Firma, MagicFPS, SignsIV, and I think there is one more free one(my apologies to the creator) but I can't remember the name of it.
Best.