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This will require a Hue/Sat adjustment afterwards to bring back the sepiatone, but if the stain is all one color:
1. Sample the stain color with the eyedropper tool
2. Create a new layer
3. Fill the layer with the sampled color
4. Invert the new color layer (ctrl-i)
5. Set mode of new layer to Color
6. Desaturate
7. As mentioned before, add a Hue/Sat layer and add the tone you want
If there are variations in tone this should still work. If there are variations in stain color, this will at least provide a helping hand.
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