Hello there.
This will be a simple Gimp tutorial on selective colouring. The technique I use is very basic, but it's very easy to do and you can get some nice results from it.
Oh, before I forget, my Gimp is in my native language (Dutch), but the options should be the same. (Even though I'm not 100% sure on the names of them)
Without further ado:
1) Import your image to Gimp:
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2) Make a duplicate image. (Either by pressing Ctrl+D or by using image->duplicate in the toolbar)
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3) On the duplicate image, select image->mode->grayscale to make it black&white.
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4) After doing this, select image->mode again, but this time choose RGB.
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5) Take the gray duplicate image and drag it to the original image, adding it as a layer on top of the original image. Make sure the gray layer has an Alpha Channel (transparency)
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6) Now, select the gray layer and start erasing parts from it. I myself prefer to use the lasso tool to select an area to delete it, but any other erasing option would work as well:
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That how it's done, the end result should look something like this:
Like I said before, this is the basic way to do it, I'm sure someone more experienced could produce something fancier, but I'm not really able to do that =P
Anyway, thanks for reading, and comment should you have any questions =)