Bit more time today. Fridays always busy for me + I meet friends in the afternoon for a couple of beers
Is Windows / Dockable windows / models. the Templates dialogue ?
You probably know most of this already, however, Gimp templates:
1. If you make a canvas then use File -> Create Template, that template will use the background colour (BG) as the fill. It does not matter if you originally used something else, BG is used. That can be changed on a per-image basis in the advanced options as the earlier screenshot.
Easiest way, if you want white make sure the BG is white.
2. It can be set permanently if you edit the Template: Windows -> Dockable Dialogues -> Templates. Select Edit bottom of the dock and change fill to White.
Little video demo of above:
https://youtu.be/ZniY6iJf9B0 duration 3 minutes