Selectively changing brightness and contrast in GIMP

Thank you very much for answering my previous question. That taught me a number of new techniques, and did a very good job of removing the orange stain and preserving the sepia tone. Now I have another question: How, with GIMP, do I most easily make the area shown in the red outline darker and more contrasty, without changing its colour? I guess that there was either light leaking into the camera, or some error was made whist developing and printing the original photo to create this lighter area. Thank you – Rowan

Where can I find a large palette / set of contrasting colors for coloring many datasets on a plot?

I need large set (20 and more) of contrasted and easily visually-distinguishable colors for coloring different datasets on a scientific plot (the set of colors should not include white or black, and colors should be as bright as possible).

How to calculate the best type colour for a random background colour?

I might need to present content at multiple levels depending on the distance of the observer to a display. Let’s say from far away the user perceives a flat colour, but from a close distance the user needs to be able to read some text. The tricky part is the flat background colour is given/can change/I have no control over.