GIMP Plugin Registry

This script simulates the 3 Color Technicolor effect.
Copy it into the scripts folder from GIMP, you will find it then under Colors -> Technicolor 3 Color.
With these options you get a 1:1 transformation from RGB- to CMY-colors.
Example here:

See also the Technicolor 2 Color Script.

Update: The sharpness layer got an inverted layer mask.
Update2: I kicked the additional multiply layers out and set better default values (now R = 2R - 1/2 (B + G)).