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.
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)).
Update3: Values back to 1.2R. New retro colors option. Example here.
Update4: Optional extra color layer.

For all scripts together, go here.