If you are talking about the two stripes that are seen on the front fender of a car, these are also called 'team stripes'. They were used to distinguish one car from the other for a team with more than one car in the same livery.
Warning of a rookie in Europe is a cros on the back of the car. It is assumed that rookies are being overtaken and should be given a wide berth!
Although I think we've now established that you're talking about team stripes....the time trial club I run with requires 3 stripes be applied to the rear of the car until you have some number of days on the track. This helps other drivers in practice to understand why a rookie car is driving so far off line or breaking in weird places.
The Rookie stripes are for the reasons stated above, warning to others when overtaking or being passed. In NASCAR, a Rookie is required to have a Red/Orange stripe along the bottom of the front airdam and rear bumper during their first season.
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