The problem with drawing a background color behind a label is that each letter in the label is actually a separate texture (and in fact duplicate letters share a single texure), and they aren’t all the same size. For example, here’s what it looks like when each letter is drawn with a red background:
This shows the actual dimensions of each letter.
One possibility to get a rectangular background behind the entire label would be to measure the overall width and height, and draw an appropriately sized billboard at the same position, behind the label. “Behind” from the camera’s perspective can be done with the z coordinate of the billboard’s eyeOffset. The Label/LabelCollection code calculates the total width/height as part of its layout, but we don’t currently expose any of that information.