There is no way to do this, at least in the Missing Manual for CC. Release the HUD shortcut keys and press x on your keyboard to switch between foreground and background, then press the HUD shortcut keys again.
Darn, thanks for the help
Well, x is close to ctrl-opt-cmd on my Mac,so it's a bit like playing the piano.
Put it in as a feature request, it might get attention.
HaHa, playing the Piano