This script simulates a photochrom image, a lithographic printing process from the 1890's:
The script is a translation of this how-to for PSE with some improvements:
Copy it into the scripts folder from GIMP, you will find it then under Filters -> Artistic -> Photochrom.
If you like it, hug someone.
An example can be found here:
For all scripts together, go here: http://sites.google.com/site/elsamuko/gimp