This is a script for The GIMP that separates an image into its component colors. The colors can be from an optimal palette generated from the input image, or from a user-specified palette.
To install it, put the file separate.scm in your GIMP scripts directory (~/.gimp-1.2/scripts). The script is accessible through the right-click menu (Image->Colors->Separate...).
When the script is executed, a new image will be created with the same height and width as the original. It will have several layers. The layer called ``Background'' is a composite of all the colors. The other layers are in black and white; the black areas are where a particular color should go.
See separate.scm for its simple, all-permissive license.