For a label attribute, when "Show label background color" is enabled, the system helpfully converts the text from black to "knockout-white" when the background color is very dark. Clients have asked for a little more control over the text color (such as, the ability to pick black or white), in case their design choice doesn't agree with the system picker.
Fig. 1: show background color
Fig. 2: Different - arbitrary - font color
Fig. 3: Picker Mockup