Well Rhys, I see you as an unpredictable mate. You see, I feel as if your underwear doesn't exactly have a color. I can assume you are currently wearing a transparent piece of undergarment. It is often seen as a tinted orange due to the sun, since you like to walk around in the nude, but it is not specifically yellow! It changes when your surroundings and environment changes, with it being transparent and whatnot!
Then again, maybe I just saw you walking around the beach nude one day and you may have had a noticeable tan line where your genitals are and I simply got it confused with undergarments!
Hope this answers your question in a humorous way enough to get your vote!