Parking Awareness Services are one of the smaller parking companies, and are members of the IPC rather than the BPA.
As part of their niche market service, they offer the option to restrict parking bays to yellow cars.
succinctly puts it, good luck with that!
The Prankster also wonders what 'membership to' actually means, but perhaps this is just him being pedantic, the English language having moved on since he last was at school and studied grammar.
The Parking Pranskter