This tweet by Mike Ashworth (https://twitter.com/Mikeashworth12/status/1534616762146332674?s=20&t=j5qJ5AxGnOLlaXYTgjGGsw) shows a BRS leaflet with details of the colour scheme used for lorry liveries, which might be interesting for people working on mid-20th century road vehicles.
He has also posted the key page on Flickr here (https://www.flickr.com/photos/36844288@N00/52131191642/in/album-72177720299650633/).
I am not sure whether it would be better to use the actual BRS colours to show different Simutrans Goods, or to use the principle of their scheme and colour the lorries using the pakset's standard colours for that class of Good.