From the documentation,
The thumbnail is an icon on a background of appropriate size. The background color is determined by the app that is associated with the file.
Combined with the following note:
Note If the associated app is a Windows Store app, the app's tile background color is used.
If your app has an association with the file, just make sure your apps Background
color is Transparent
(not 100% sure you can do that, actually).
The other option is to use something like WriteableBitmapEx to find every pixel of the tile color, then turn those pixels to Transparent
. Shouldn't be too absolutely terrible, but will have an overhead (naturally).
Hope this helps and Happy Coding!