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Try Sudsy Lemon Ammonia. Fill the washer with warm water, then add the ammonia, let it mix into the water, then add the pants.

If that doesn't work, try a cup of bleach using the same method. Be sure that all the ammonia is rinsed out of the washer and pants before using bleach because ammonia and bleach mixed make a noxious gas.

You didn't say what the fabric is. If there is synthetic fabrics, you won't get that fade.

Good luck. The pants will probably come out blotchy instead of a uniform color. Still, if you are willing to sacrifice the pants, give it a shot.