They can't be checking for Jet A1. This has no dye or identifying colourant.
More than likely they are checking for red diesel. Some vehicles which never go land side are allowed to use the airside pumps. These have a dye included in the fuel to prevent misuse. (they are tax exempt/reduced)
Allowed exceptions include baggage trucks, beltloaders, etc.
Interestingly during the fuel strike a few years back an exemption was introduced to allow essential workers (which included airport staff, engineers, flight crew, etc) to legally use the stuff.