The yellow stains aren't difficult to remove since they are of a very uniform color, which is pretty close to pure yellow. In other words, in the yellow stains the Blue channel has a very small value, instead of being identical to the Red and Green channels. So to remove the stains, we need a Blue channel which is equal to the Red or Green channels. So kick off Color>Components>Channel mixer, leve the Red and Green channels untouched, and set the Blue channel to 100% of either the Red or Green (or 50% of Red and 50% of Green).
PS: Btw, looking at the histogram, it seems the image has been heavily processed already. Can you show the original?
PS: Btw, looking at the histogram, it seems the image has been heavily processed already. Can you show the original?