This script simulates a photochrom image, a lithographic printing process from the 1890's:
If you like it, hug someone.
Update1: Slightly different contrast layers. New retro option which
replaces the orange with a yellowish color.
Update2: Optional b/w dodging. It also deactivates the extra color layer. Thx to Francois for the tip.
For all scripts together, go here: https://sites.google.com/site/elsamuko/gimp