It's not. It's sweat. Never seen yellow sweat before.
It entertains me that I just read a passage in my Firefly companion books about how the girl that was modeling the huge pink dress in the beginning of the Firefly episode "Shindig" apparently "sweat acid" and turned the armpits of the dress a horrid yellow, which they couldn't repair because the dress itself was a re-sewn antique Indian thing. If I were Jewel, I dunno if I could've handled putting that on afterward.
