Lighting error on signs

While in your inventory, if you position your mouse on an empty inventory slot, (also works in the 9-slot hot bar!) visible signs on the sides of the screen appear as a light tannish color. However, when I positioned the mouse on a taken inventory slot, the signs turn a slightly darker shade. The signs return to the lighter shade immediately upon exiting the inventory.

What I expected to happen was...:
The signs would stay the same shade of color.

What actually happened was...:
The signs turned a darker shade of color.

Steps to Reproduce:
1. Place a sign, back up, and go in your inventory (make sure the sign is visible in the background!)
2. Aim your mouse pointer on an empty inventory slot, and look at the sign.
3. Aim your mouse pointer on a full inventory slot and look at the sign again.
(Works in all game modes!)