When I invoke this system dialog, it is in HSL mode, not RGB mode.
And it is not obvious how to get RGB values out of it while in the dialog. If I choose a color from the right hand panel in HSL, it returns (0,0,0) in RGB values.
I have to choose something from the Custom Colors to get RGB values to show. From that point on (and for each subsequent invocation of the dialog), RGB shows for right-hand panel selections.