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.