Thanks for the example. I found a bug in the Inspector: colours left at Default could show a different value from the one used by the designer preview.
I’ve fixed this for FillColour, TrackColour and TextColour on both RadialGauge and MeterBar, including switching between light and dark previews. Custom colours are preserved. The fix is tracked here for the next update.
There is also a theme difference in your screenshot. The printed #FFFFFF99 is the dark-theme default, while the designer is previewing light mode. The sun/moon buttons change the designer preview; the running application follows the system appearance.
To choose a fixed value instead of the theme default, assign TrackColour explicitly.
