From my Life Drawing Sketchbook

 Drawn with Tombo markers.

 Drawn with a Tombo marker.

Model reference is from Pixelovely, drawn with Tombo markers.  This is an observational exercise I do quite often.  I first sketch the silhouette of the model with one color and then use a second to draw in the shadows.  If I drew the silhouette right then I should have no problem adding the details.

These three Master sketches were drawn with a Staedtler Mars Graphic 3000-love this marker!

Model reference is from "Figures/Faces" by Hugh Laidman, drawn with a Tombo marker.