If nothing else will work I suggest you try bleach. Yes, bleach. Many years ago I had some lovely, very old baby dresses. They were made of handkerchief cotton with smocking and beautiful embroidery. But they had old "age" stains from storage. After trying several methods I finally decided they were useless unless the stains were gone.
So I hand washed in a bleach solution and soap. I think I used the regular amount for laundry. Well, they came out great and the colored embroidery thread wasn't affected at all. (old embroidery thread was processed with lead somehow and I think that's why they didn't fade in the bleach).
You might want to try this with one or two pieces to see how it goes.