What is the purpose of the Red (Supply) and Yellow (Service) pneumatic coupling lines between a truck and a trailer?

1. Red line = service-brake control; Yellow line = continuous reservoir supply
2. Both lines supply reservoir pressure; the electrical connector alone controls service braking
3. Red line = pneumatic parking-brake release only; Yellow line = trailer suspension supply only
4. Red line = continuous supply line charging the trailer reservoirs and supporting emergency action; Yellow line = service-brake control line whose pressure follows the braking command