1. Click the eye icon on the LinesShadows layer to switch it back on.

2. Right-click this layer and select Add Layer Mask.

3. Select White (full opacity) and click Add.

4. Select the mask (the white rectangle that's appeared beside the layer).

5. Click the Paintbrush icon.

6. Select a large rough brush. Don't select anything too round or smooth. I like the "Galaxy, Big" brush that comes with GIMP.

7. Hold down Ctrl and rotate the middle mouse button to zoom in a little on the focal area of the drawing.

8. Quickly and roughly paint in the most important areas.

9. In the Paintbrush pallet, change Opacity to around 50.

10. Paint in some more areas.

11. Reduce Opacity to 20% and paint over any larger areas of color to simulate a watercolor wash effect. Go over some areas more than once to build up color.