What is Color Spotter? Basically, it's contrast, but "centered" around a color. Another way to see it is as another "Color selection" tool: you specify an image, a kind of threshold, and it selects color that are close to the one you specified; only, Color Spotter keeps every pixel from the image, darkening the wanted colors, brightening the others.
The best way to understand is to see it in action:
Here, the selected color was from a pixel from the plantpot (beige-ish).
This plug-in was mostly an experiment for me: you could say I made it just to get familiar with programming C plug-ins in GIMP. I have another plug-in I want to make, but as it's much more complicated, I wanted to make sure I could deal with the basics first.
Suggestions welcome! Thanks for reading, hopefully this will be useful to at least one person :)