Similar story to Suz. I wore my white jeans for a movie, and I guess someone who sat on that seat before me dropped ketchup on it. Anyways, you can't see all that much in the dark, so I sat on it the entire length of MI5. When I got off the car at home, my husband told me that the back of my thighs were stained. And what a stubborn stain!!!
I tried 3 kinds of stain removers but they didn't work. Finally I soaked overnight in fabric whitener and then line dried my pants in bright sun. The stain remover didn't lighten the stains, but strong sunlight is a great bleaching agent! The stain lightened a bit....and then I did the same thing again....and now it's barely noticeable.
So wash your pants, and put them to dry in the strongest sunlight, with the stain facing the sun. Hope it helps!