This is my first script-fu plugin. It is an adaptation of 3 different methods for muting the colors in an image.
1. A simple copy of the existing layer, desaturating it with luminosity, and setting opacity over the original.
2. One of my methods for producing similar results as described by the originator (Flickr user Tennessee_Gator):
This method uses 2 luminosity layers with different blending modes (multiply & screen) over the original color layer to produce the effect.
3. A different method to the same effect as (2). This one uses solid fill color layers (black and white), and uses a luminosity mask on each layer.
The command can currently be found (after install) under Colors → Mute Colors