colour is one of: black, white, grey, red, green, blue, and yellow.
Alternatively one of the additional named colours listed for %bg can be used.
colour_value is typically a value returned by RGB@. If the grave accent
is used, this value can be changed after the button has been displayed. colour_value
is then passed to window_update@ in order to update the screen.
%bu is similar to %bc but is used to specify the foreground (text) colour.