Here's a quick explanation, hope it helps. Of course, ask if anything isn't clear.
The easiest way to make your own numbers is to pick up the default files and mod over them found in YourDCSFolder/CoreMods/WWII Units/FW-190D-9/Textures; from there you can unzip the files and grab one of the number files. The template is always the same, so which one is irrelevant.
Important to note is that the number files have an alpha channel (In which white = opaque/black = transparent and any gray shade in between to mix)
As it appears with the description.lua above:
The colour file, also: