There are some black gas stoppers, but an easy way to check before assembly is to immerse the stopper in a fuel-oil mix. If the stopper develops a "slimey" feel after 10 minutes or so it is not gas compatible. Better to purchase one that's specifically noted to be gasoline compatible. Doing so may save you an airplane where a non compatible stopper can degrade to grossly unacceptable levels on the first flight.