These are some more experimentation with digital collage.  

1. original image, touched up brightness/conmtrast

2. image -> adjustments -> threshold to make it into a stencil.

window -> channels, select Blue, then click on the circled button (load channel as selection)

3. back to layers, double click background layer to convert to a normal layer. click delete to remove the white of the background

4. color on a new layer below the stencil

I like this technique because it's very versatile. You can take the stencil as a line art and pay on top or underneath it. I used the lines on the examples above to put in some more colors. A way to achieve the same but with traditional materials, would be to draw the silhouette on top of a collage.