Actually… 
And it was in plain sight, in that topic about hidden formulas. Props to @Filmos for finding the list of valid color values and for tinkering with the Button() function.
Important: other color values will crash the doc. Partially typed in values (e.g. when you start typing "Purp" within quotes) will also crash the doc. You won’t be able to recover it unless you open another section by link and delete the section where the incorrectly setup button is displayed (i.e. avoid seeing the button and triggering the crash again)