Strange Background Appearance In Tkk.Frame

I discovered an anomaly in tkinter. I have attached my script below to show the anomaly. Running this script, I found on my system (Ubuntu 16.04.03) that the background of the ttk.

Solution 1:

On Win 10, 3.7.0b1, no background given results in white, a recognized background is what it is, and an unknown background ('light purple' or 'xyz') is black. As for recognized color names:

The NMT 8.5 reference says "The colors 'white', 'black', 'red', 'green', 'blue', 'cyan', 'yellow', and 'magenta' will always be available. Other names may work, depending on your local installation.".

The tcl/tk 8.6 docs list hundreds of color names (but not 'light purple'!) plus some specific to MacOS and Windows.