Green ADI Pink PDI

An ADI is a registered driving instructor and known as an Approved Driving Instructors or ADIs for short. They are required to register themselves with the Driver and Vehicle Standards Agency (DVSA) and may also self-register with the online ADI driving instructor list maintained by the DVSA. All ADIs must display either their ADI certificate, which is green, on the windscreen of their tuition vehicle.
You may come across the term PDI which is a Potential Driving Instructor. A PDI is also known as a trainee instructor PDIs can deliver driving lessons whilst they work towards their final test. A PDI must also display their trainee driving instructor licence, which is pink, on the windscreen of their tuition vehicle.
Warning: If your instructor is charging you for lessons but doesn’t have a trainee licence or the full certificate, they are breaking the law.