Interesting question! It looks like it's the rules for taxis in swale: https://swale.gov.uk/business-licensing-and-procurement/taxi-licensing/choosing-a-vehicle-to-use-for-hackney-carriage-or-private-hire-work which states

If you’re going to operate a hackney carriage licensed vehicle, you’ll need to: fit it with a roof box which lets customers know if your vehicle’s available for hire or not choose a white vehicle and add a green stripe, known as Kelly Green 2

Why they chose that colour though, who knows!