GIMP :: Ancient Manuscript Ink Differentiation?

I work with old manuscripts, many of which are digitised and available online. However one of the problems our readers have encountered is differentiating the main text from ink which is coming through the page from behind (bleed-through), and ink which has come from the page adjacent (offset). What I am hoping to achieve is to have one final image with the main text in one colour, and the bleed-through and offset in other colours so we can quickly identify what is real and what isn't.

I usually work with ImageJ but am hoping to use GIMP to assign colours to each of these layers. I have three base images to work with (the front of the page, the back of the page horizontally flipped, and the adjacent page which is causing offset). They aren't registered, but even something like using semi-transparency on an overlay, and physically colouring in the different ink areas could work, but I'm not sure where to start.