You are advised to learn all the flags because if you do a sprint at a circuit, all flags will be in use. The two most common flags on Championship Rounds are:
Green Flag: Occasionally used at the start line. Go!
Red Flag: In Hillclimbs and Sprints, if you see a marshal either holding out or waving a red flag STOP straight away and pull over to one side of the road making sure to leave enough room to allow Services (Ambulance/Rescue/Officials) to get by. Turn off your engine and wait in your car with all of your safety gear on. DO NOT attempt to turn your car. DO NOT drive back down the hill unless instructed to do so by a Motorcycle Marshal or Official Course Car. A flag marshall cannot tell you to return to the start line. If you have been red-flagged on a timed run, you will be allowed to restart your run.