Creating a Partially Black and White Image

There are two ways to do this.

Original Image.

1) Duplicate your layer. Change the top layer so it's completely black-and-white and erase the part you want to be colored. It will look something like this (my example is really sloppy - you will, of course, do a much more detailed job).

NOTE: Sometimes the newest version of GIMP goes wacky and decides when you erase that you really want to turn everything white. To fix this, create a new transparent layer and merge your top layer into it (right click > Merge Layer). Then you can erase to your heart's delight.

2) Using Hue-Saturation, click on the colors you don't want to show up (in my example: blue and cyan) one at a time and move their slider thing to -100. You image should look something like this.

NOTE: This only works where you have very distinct colors in your image. It will not work for images that are basically all red or all blue, etc. Your image will look like this if it isn't distinct.