If you use an image with a transparent BG color on a label, the transparent color will only be updated when the image is initially applied, or if you re-open the window, changing the BG color of the interface control will not properly update the transparent coloring.

Testable Image: http://www.byond.com/games/hubicon/86639.png
Step 1: Create an interface
Step 2: Add a label control
Step 3: Change the label BG color to black
Step 4: Apply the image
Step 5: Chang the label BG color to anything else
Step 6: Fail