Absolutely, my salesman order book(s) 1949 and 1950 clearly state that the zone offices can accommodate the requests. You will also notice it is referred to as an option not an accessory. Options denote factory, accessories are a dealer applied part or upgrade.
https://www.flickr.com/photos/advance-design_parts_co/8335509814/in/album-72157631872034068/
With no codes on the plates you could paint your AD any color and make a case for it (within reason for that time period).
A little later version but this GMC salesman chip book lists over 200 colors not shown in the data book.
https://www.flickr.com/photos/advance-design_parts_co/25524289148/in/album-72157631872034068/
https://www.flickr.com/photos/advance-design_parts_co/25524280748/in/album-72157631872034068/
What they advertised in 1942, fleets were big business for GM.
https://www.flickr.com/photos/advance-design_parts_co/43741368201/in/album-72157631872034068/

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr