How to remove a color from a flattened JPEG and preserve what is below?

[Azhrei]

(Hopefully this is a new question. I searched the forum for "removing color" and similar phrases but didn't have any luck.)

I have a map that I've copy/paste'd from a Paizo PDF (for CotCT, for anyone who's interested). The problem is that the original JPEG from the PDF has an overlay on top that I'd like to remove.

It appears that the original map had a layer with two straight lines that join at a 90-deg angle in a light color. When the JPEG was flattened, the semi-transparent layer with the lines obscures some of the ground underneath.

What I need is some kind of filter that will subtract out a given color value. I tried using GIMP's Color to Alpha... but that didn't work. It made the light color transparent but removed the color from underneath. (Hmm, maybe with the right ground pattern underneath the original image, the transparency effect might be enough?)

Here's a screenshot of the corner that shows what I'm trying to accomplish.

Maybe I could work out some way of mapping colors from the obscured area to an existing color somewhere else on the map? So an obscured pixel with a certain color value could be mapped to another pixel color, then all similar pixels would be changed at once. But this seems awfully tedious to define and subject to error since the image is a lossy JPEG.

Thanks for any help/suggestions!