Here's how I'm shading and lighting the walls for my game.
I was considering using Screen or Additive blending for the light, and Multiply or Subtractive for the purple shadowing, but since I have the overlays so low in transparency, it seemed regular alpha blending had the light and shadow coloring showing up better better.
While it technically doesn't make sense for there to be arbitrary lighting on one end of the wall and arbitrary shading on the end, it adds more "life" to the wall, I feel, making it look less flat. And, as an artist friend was saying to me, 'The artistic rules with lighting is basically that you get to make it up, because what with light scattering and light rays bouncing around everywhere, you can pretty much just do whatever looks good and get away with it.' (paraphrased)
With lighting, you can just cheat as long as it doesn't stand out so badly that it draws people's eyes to it. With this wall being in the midst of a scene with many things to look at, the subtle lighting at the ends won't be too noticeable, but will still add to the overall visual aesthetics.
Probably, maybe, hopefully?