Each layer is built up exactly the same. For this picture, I used a layer for the face (as above) another for the jacket including the medals, another for the hair, and a final layer for the background.
***NB. Before creating a new layer, make sure you click on the Background layer in the layers pallet. If you don't you will just create a duplicate of the last layer you were working on, not the whole picture. (Thanks, and apologies Keith)***
The nice thing about doing it this way is that if a layer looks too bright against the other colours, it can be softened by simply reducing the Opacity in the Layers toolbar. It saves a lot of hassle of re-selecting and re-colourising. Or conversely, the saturation can be increased to make it brighter without affecting everything else.
Once everything looks okay, go to Layer -> Flatten Image and then SAVE IT. Many's the time I've coloured a picture and got to the last stage, thinking "Phew, thats done!", closed the program and not accepted the changes! I think that is the situation swear-words were invented for!
Any questions, please let me know.