It speaks for itself really.

So I made a resource pack that changes a bunch of things to red and I couldn't help but notice this.

All I did to the banner texture was change the stick the banner was on the red. I didn't change the color of the actual banner.

Also I made a discovery that it's not just Ominous banners, its ANY banner that has ANY design on it. The banner will just become the color of the last design you put on it. So like if I put a red Creeper charge on a white banner, the entire banner will turn red.

(Don't mind the pillager, inventory, environment, etc., they look like that from the resource pack)