".....they are commonly issued to new vehicles for use during the eight weeks or so that it takes for new plates to be issued.
Trade plates carry a significant number of restrictions. Drivers with red plates must seek permission from authorities if they want to use the vehicle at night. Cars with red plates cannot be driven out of the province where the vehicle was registered at purchase. Drivers must ask for permission each time they want to travel outside the specified area. Documents must be in order, with the destination and the dates of travel. Red-plated cars are not allowed to use expressways. Drivers of cars with red plates must have all car-related documents, a proper driving licence, and ID card ready for checks at all times. Police officers are authorized to stop any car with a red plate to examine the car and the driver's documents even if no traffic offence has occurred."
"As of July 2015, DLT has a plan to stop the use of trade plate as temporary license plates, and use a quicker car registration process."
https://en.wikipedia.org/wiki/Vehicle_registration_plates_of_Thailand
I guess things move slowly at the DLT Thailand!