rhm

You have encountered the answer to the age old question, "why are cycling shorts black?"

I'm afraid once this has started to happen, it's hard to make it stop. The saddle was too dry, so you made it wet, and now there's dye (or the functional equivalent of dye) suspended in whatever solvent you put in it, and it will transfer to any absorbent material that comes in contact with it.

I suppose it might be possible to soak the saddle in water, changing the water repeatedly until all the dye is gone; then drying it out again and replacing the lost moisture with oil of some kind; but adding oil might cause the process to repeat and anyway, none of this is going to do the saddle any good. I don't recommend even trying it.

Letting the saddle dry in the sun for a loooong time might do some good, but will not solve the problem 100%.

When you ride with good pants, or any that will show a stain, you have to put a plastic bag over the saddle. This will compromise comfort to some degree, but it will protect your pants. Or wear black pants!