I have a couple of additional comments about editing the banners that appear on the strategy map (displayed on armies and settlements).

The alpha layer behaves strangely in with these banners. It does not seem to be possible to create one with a white crest (logo), for example. If you add a completely white silhouette to the alpha layer and leave the RGB layer blank, the crest will be rendered in-game as a medium grey, as with the Senate banners.

Coloured crests seem to interact with the underlying colour of the banner, which is defined (as the primary/secondary colours of the faction) by settings in the file: "Data\descr_sm_factions.txt".

In my attempts to place a light yellow crest on a dark blue banner, the crest was always rendered in-game as a dirty green colour. I settled for grey in the end. :)