As I was working in GIMP, I found myself having to draw an area with a color then use Colors/Invert then use the color picker tool to get that inverted color.
I wanted a quicker way to simply invert the working foreground/background color and I didn't see a way to do that so here's a simple script that helps with that: http://www.gimplearn.net/viewtopic.php/ ... f=3&t=1482

Here's another idea (haven't checked your code, if it does something similar):
http://www.mattlag.com/scripting/hexcolorinverter.php

It's the same I think.
newR = 255 - oldR
newG = 255 - oldG
newB = 255 - oldB