Also on another note I failed to make this.
Adds like double the options of colours and slightly rearranged the positions of the menu so you can properly select all the colours. The second version of my BannerEditor Enhancer Mod.It seems like it shouldn't be this easy, but it is: just press Ctrl-V to paste in your banner code When you're in map view, pressing B will bring up the banner creation window.
Hit CTR + V to paste the code into the game.
Home Banner editor Bannerlord How to create your own custom banner in Mount & Blade II