This looks to be a Garmin firmware issue, as this feature works properly on the simulator. (It seems that the Connect IQ library call which determines the data field background color isn't working properly on the physical device.)

Thanks for the bug report! I'll get this reported to Garmin.

For transparency, here's a link to the bug report: https://forums.garmin.com/developer/connect-iq/i/bug-reports/datafield-getbackgroundcolor-returns-color_white-when-background-is-black