Thanks for the detailed explanation. But I still think it's a bug. Why would there be three different looking mundane large shields for sale, with different colored interior panels on the inventory art, when all of them change to your character's color when you equip them? That doesn't make any sense. Why not just have one mundane large shield if you are going to have it become the character's color?
If the tan magic shield thing was deliberate, then I think it was a terrible design decision. More likely, they forgot to color in the three mundane shields so they match their inventory appearance, but as it happens this results in better looking shields in-game.
I guess I will make a Tweak request for this if you don't regard it as a bug. If you can tell me how to mod the magic shield file myself to make it take on the character's color, though, I would be happy with that.
Not sure what tools you are using to look at these fields and values for items. I am familiar with hex editors. Can you tell me how to make magic shields change colors like mundane shields?