I don't know if this is applicable but, when I was growing up, there were no synthetics except for nylon. So I remember sweat bleaching out colour (though that was generally under the arms). It could be the same principle with the pants, especially if they're of a good quality. Have you heard about trouser slips (that's the only name I can think of)? They're trousers made of nylon that you wear as a lining underneath your trousers. The first thing they do is protect your skin from the possible harshness of the fabric. The second thing they do is prolong the life of the clothing. Because, if you sweat or bleed through, you're going to be able to throw the liner into the washing machine.

The only other thing I can suggest (because I sometimes get white patches in the crotch area after wearing my fave cotton/lycra leggings) is to blot the area with COLD water.