I believe it is just remains from the gas evaporating. It seems that most gas evaporates into a clearish whitish yellowish crystalline remains. If there is enough gas it seems it ends up being the green gunky stuff like in her pictures. Then I find if it's a little more volume a fuel and longer time it ends up being a darkish red molasses thicker concoction. The only thing I think you find that's not from the gas is in some older aluminum carburetors the aluminum starts to oxidize. This is mainly the parts that are dry near the top of the carburetor. This ends up being a white grayish powder that sluffs off as it eats away on the carburetor.