The plakettes are indicators of the testing of the vehicle, with the general test one (circular) on the rear plate, and the engine emissions one on the front (hexagonal). The year of test is indicated by colour, with 5 colours cycled in order, Orange, Blue, Yellow, Brown, Pink and Green.
Orange is 2007, but also 2001, 1995 and so on. We added Red, and Silver Grey, just for fun!