Britain and the USA used letter codes for their aircraft, most of the time, at least in the European theater. Numbers would be used for USN aircraft.
The Germans tended to use numbers for their fighters, at least later in the war. For their bombers and twin-engined aircraft, many times they used letters for the Gruppen, Staffeln, and individual aircraft codes. Different colors were used for the above.
I'm not that familiar with Italian aircraft.
There are a number of decent books on the subject, but can't recall any websites right now. There are some, but just don't remember their names, or addresses.
Try a search on the nationality desired, and "markings" "WWII", or "WW2", and "aircraft", or the plane designation(s) you are looking for.
That should turn up something.
I hope that helps.
Best regards,
Rob