Since 2015 the FIA regulation has been clear and states that a driver could use a different livery design on one occasion than the standard design.
This rule was originally introduced to help the audience identify drivers more easily and avoid cases like Sebastian Vettel's one who, when he ran with Red Bull, changed the livery of his helmet for each Grand Prix.
If we go back to last year, the episode involving Daniil Kvyat comes to mind. In fact, at the Sochi Grand Prix, the Russian driver wanted to wear a celebratory helmet for his home race, but the FIA had forbidden it, since he had already used one with a different design than the default one during the Italian GP.
But also Max Verstappen changed his helmet quite radically on at least two occasions on the 2019 calendar.
This is what the rule stated: “In order for drivers to be easily distinguished from one another whilst they are on the track, the crash helmet of each driver must, with the exception of one event of the driver’s choice, be presented in substantially the same livery at every event during a championship season.”
However, the FIA World Council has decided to liberalize the design of drivers' helmets, which will therefore be changed without limitation by 2020, but this decision was officially approved by the WMSC, which noted that from now "unlimited changes to the driver helmet models between races" will be allowed.
Furthermore, this decision could also arise from the fact that with the introduction of the halo the driver's helmet is not as easy to recognize.
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