[Following up on #44243.]
tk/tkinter-based turtledemo buttons are currently white on something.  On Mac, configured foreground button colors are honored (while the button is unpressed), while background colors are ignored.  Given that the unpressed default is black on white, the configured result of white on something is white on white.

I don't like the current colors anyway, so I think we should change to something on white.  I am thinking of green, red, and blue for Start, Stop, and Clear.