I don't know about everyone else but, I think its hard to find tutorials for anything other than Photoshop. Not to knock Photoshop or anything. Photoshop is really nice if you can cough up the money for it. So I went the less expensive route.
With just a few easy steps I will show you how to use the clone tool to add color to a black and white photo.
Be sure to save your photo under a new name so that you don't mess up your original.
What you will need:
1. The latest version of GIMP. http://www.gimp.org/
2. A color photo
3. Steady mouse hand
Step 1: Start Gimp and Desaturate
Then open a color photo of your choice. Be sure to "Save as" before you modify the photo. I named my photo "flowerblackandwhite" Once opened, select "Colors" from the drop down menu and choose "Desaturate". This will convert your photo into shades of gray.
Step 2: Open a Second Copy of Photo
Step 3: Selecting the Source to Clone From
Then press and hold down the "Ctrl" button and select an area on the color photo that will be easy to find on your black and white copy. This area should be in a spot you would like to add color to your black and white copy.
I always try to find a spot in the photo that is easy to pick out so that I will have a good reference point to start from for the next step.