There's an even easier way to do this, if you're curious.
What you can do is ...
1. Open your original image (obviously, haha

2. Duplicate the base layer.
3. Desaturate the duplicated layer. I'm not wholly sure where the desaturate option is in The GIMP, as I'm using Gimpshop; in Gimpshop, it can be found under "Image --> Adjustments --> Desaturate." It may be somewhere else in The GIMP; poke around until you find either "desaturate" or "Hue/Saturation."
4. Erase the part where you want colour -- the base layer is still in colour, and will show through.
Ta-da!
Er, sorry if the instructions weren't very clear ... I can try to explain more if you want to try it yourself later.
(Yes, I know it's a late reply, but I figure if it helps someone else out...)