This is just a guess from my Tk experience, but some widgets use -bg instead of -background.

Some widgets yes, but not ttk::button

A "tk::button" aka a button does take both -bg and -background

ttk widgets, themed tk widgets, manipulate that stuff through -style , kinda like css

see http://www.tcl.tk/man/tcl/TkCmd/ttk_intro.htm, http://www.tkdocs.com/tutorial/styles.html

Thank you. I would seem to be constrained by the settings of the style sheet associated with the widget (and I'm not so desperate for specific colours that I'm going to go revising the style sheet just for fun - there's a job to be done behind all this :) ).
Those links were very helpful and led to an interesting discovery which appears to differ between ttk and Tk:
A widget state of active != !disabled. At least not when the Tbutton style applies. I wanted to disable and enable a button during various processes but setting the button's state to "active" did not re-enable it.
$w->state('!disabled') reverses the effect of $w->state('disabled')
$w->state('active disabled') is not an oxymoron but it makes the button rather difficult to press while both states exist.