In the game, when decorating a building, I want to draw motifs on the walls to beautify the inside of the house, but when there is only one row of walls, it paints the exterior. It’s not supposed to be like this. The double row wall is also very thick and sometimes looks ugly so I don’t do it. right? When are they going to fix this, or can there be a mode about it? 
I’m sorry if this was raised earlier. I couldn’t figure out how to search it😢
Because you paint blocks not walls
I know, but why are we painting every inch of a block at the same time? Why can’t we paint one face? That’s what I mean.
It’s the unfortunate nature of cost/benefit assessments - the cost in time and effort to build in the extra functionality to 1) have each face of a flat block have its own graphical values, and 2) to change those values in the GUI at either the individual or room wall/floor/ceiling levels, was probably seen during the initial game development as not worth the benefit (value for given consumers). There would also be data considerations during loading and saving of assets. It’s probably not something that could (easily/at all) be retrofitted into the game by any modder.
Indeed, it’s how the game is made on a foundation level - the world is made of blocks and blocks have a single, flat color. Such a change is probably impossible to be made perfectly.
What a mod could more easily achieve, however, is maybe create some “painted plaster” wall decorations that basically are flat surfaces that you can then “attach” on walls inside. Not ideal solution, sure - but can be done and would work with current mechanics.
They could be offered in multiple sizes like 1x1, 3x3, 5x5, etc… allowing you to cover more walls with less parts
Yes, that could work. I suppose the game is very similar to minecraft in many ways. Although from the perspective of villagers.