I was told that the red plates are dealership plates - see also
Trade Plates:
Vehicle registration plates of Thailand - Wikipedia, the free encyclopedia. It allows the dealership to transport vehicles temporarily (i.e. from port to dealer, main dealer to branch - you get the idea.) The department of transportation only issue a certain number of these plates to each dealer (all with the same number for as far as I know), so said dealer can really only have as many cars being moved around as he has sets of plates. These plates do have the official Government seal to make it difficult and illegal to copy them. However, there are official rules with regard to the use of red plates but this being Thailand, the men in brown have many ways to interprete these rules:
Red Plates:
Can only be used from 06:00 to 18:00, though there is a police directive to extend this to 20:00 in situations of heavy traffic - max fine: 1,000 Baht.
Cannot be driven outside of the province indicated without special approval of the Land Transport Department (permission must be noted in the log book) - max fine: 1,000 Baht.
Logs of each journey must be kept - max fine: 1,000 Baht
Cannot be used on the one car for more than 30 days or 3,000KM - max fine: 10,000 Baht
In addition, the use of fake red plates will result in a 6 month to 5 year jail term, and a 1,000 to 10,000 Baht fine.
Some owners of luxury cars do seem to like perpetual red plates though (was told it is a kind of status symbol as red plates are used for brand-new cars), knowing that they won't get bothered at checkpoints except for the usual "tips" to the men in brown. Maybe more important is
that their car only starts officially ageing from the date it gets white plates.