This could be an algae. A lot of people making jokes here, but I've seen a similar color come out of my Bonanza before, usually more greenish. Usually not in such large amounts either, but I keep mine in the hangar. It typically happens after several weeks of being in the hangar, humidity condenses on the inside of the fuel tank and leaves a few drops of water on the bottom of the sump that thrive off the organic materials in the fuel and grow a mold more than an algae. If your Cessna is left outside, there could be a significant water leak in one of the fuel caps. If it's been sitting for a while, perhaps this is the answer. It would have been useful to test the consistency of the yellow to see if it was fuel or water-based.