Another function available to you using the Gimp is that of the colour fill. This technique is most effective when used on a black and white image. You can either create a specific black and white image with the camera or use the greyscale tool in the gimp to prepare one.
Once you have an image that you wish to work with, use the bucketfill control to alter the colours of specific regions. The image I have given as an example is an extreme one, but you can be much more subtle if you wish.
Steps
1.Open image in The Gimp.
2.If it is a colour image make it a greyscale one by using the greyscale tool in the Gimp. This can be found by right clicking your curser over the image. Then scroll down to image, then mode, then click on greyscale.
3.Then you can begin to work, click on this icon
which will allow you to begin adding areas of colour to your
image.
4.To selct your colour click on the upper of the two
squares on the icon that looks like this,
this should bring up a pallet of different colours.
Experiment until you are happy. Click close.
5.Place your curser over the area of the image you wish to colour and click the left mouse button. Then repeat. To change your colours repeat step four.
6.Save your image.