Why are we unable to change the color of the left and right start menu panes independently of each other?

Why is it hard coded to white in windows 7 style on Start 10, and hard-coded to be unable to use a separate color on either side in Modern or Windows 10 styles?  Or am I missing something?  Can Windowblinds do what I'm asking about because I am unable to figure out how to do it with Curtains.

(For reference, I am referring to skinning, which I am capable of doing and have been working on for a while now, not just using someone elses skins and tweaking settings in Start10 or Curtains).

Examples of what I mean:

(Please ignore my sloppy recoloring, I wasn't trying to win any art contests here, just show what I meant).

Is there a way to do this in curtains using different layers, or with the painting margins or any other way?  If not can WindowBlinds do it?  I'd like to keep working on my skin, but this question is bugging me.