According to the package, the yellow is High Density PTFE tape, and the package says it is for threaded joints handling gasoline, petroleum oils, propane, butane, naptha, benzene, kerosene, and natural gas.  According to my local natural gas company, standard teflon tape does not hold up well under these conditions, thus the yellow stuff.  A lot of professional installers still stay away from teflon tape entirely for gas applications because of the problems with the white stuff.

Bill

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