That's got to be very frustrating!

I had some brown mystery stains on a quilt that sat unfinished for 23 years, and when repeated washing didn;t work, I saturated the stain with lemon juice and out it outsude in the sunshine. It faded them to the point where you really have to strain to see them.

This was a cream colored fabric though, I don't know if it would work on any another colored fabric.