Last fall my cheapo Champion tights bled onto my white leather Fizik Arione over the course of a 120 mile ride. I noticed it looking quite dirty at the first food stop, at maybe 15 or 20 miles. It looked horrible by the end. I tried Magic Eraser, a bleach pen, saddle soap, Oops!, Simple Green, an Oxy-something something,.... The only thing that worked was Shout gel. It took repeated applications. I'd slather it on, wait overnight, clean it off, and start over. Sometimes I'd let it dry out thoroughly for a couple of days, because it was fall and cool out and not the greatest drying weather. It helped A LOT. However, I stopped short of really really white, because it started to raise the grain of the saddle, and I was afraid it was starting to lift the white off the saddle. So it looks a little dirty, but MUCH better than before.

I think the problem is that it is dye, and not just dirt. Now I'm afraid to wear anything on that bike that I haven't worn on it before.

Good luck! Personally, I'd go straight to the Shout, but your saddle may vary. Is it leather?