Restart your PC once the installation process is complete. Launch The Gimp by double-clicking the icon or selecting it from the Start menu. Now open up the image you wish to selectively colourise via File, Open. Duplicate the image by pressing Ctrl, D or choosing Image, Duplicate.
Now we'll remove the colour from the duplicate. Convert the image to black and white by selecting Image, Mode, Greyscale. Now change it back to RGB by selecting Image, Mode, RGB from the top menu bar - this won't restore the colour but will help the black-and-white sections to match up with the colour areas.
Go back to the colour original and open the Layers dialog box by pressing Ctrl, L or choosing Dialogues, Layers from the top menu bar. Click on the ‘New layer' button at the bottom left of the window. In the New Layer dialog box type ‘B&W' to name the layer and hit ok. Select it in the layers window by clicking on it.
Go to the black-and-white image and select it all by choosing Ctrl, A. Copy it by pressing Ctrl, C or Edit, Copy. Go to the colour image, making sure the new ‘B&W' layer is selected, then hit Ctrl, V or Edit, Paste. The black-and-white image will be pasted on to that layer, covering up the colour background image.
Click the anchor button in the bottom of the Layers window and close the black-and-white image window. In the Layers dialog box right-click the B&W layer and select Add Layer Mask. In the Add Layer Mask options dialog that appears select ‘White (full opacity)' and hit ok.
Now we'll ‘paint' transparency into the black-and-white layer. Make sure that black is selected as the foreground colour in the toolbox and that you've clicked on the layer mask part of the B&W layer (the white square to the right of the image thumbnail) to activate it. Choose Dialogues, Brushes and select a fairly large brush.
Select the paintbrush from the main palette of tools - it's the one labelled ‘Paint fuzzy brush strokes'. Then simply paint the parts of the image that you want to be coloured. Repeat with a smaller brush size for the fiddly bits. If you're looking for a harsher edge, try the pencil tool (to the left of the paintbrush).
Finally, crop your image for maximum impact. Select the tool labelled ‘Select rectangular regions' - it's in the top-left of the main palette. Click and drag around your chosen area of the image. Once you're happy with the crop, choose Image, Crop Image to remove anything outside this area.