While I was at Manchester Museum, I ‘grabbed the chance’ to do an exercise I had missed out earlier.
… take a look at the darkest areas only. Use coloured pencils or pastels and sketch them in roughly. Then, using a different colour, sketch the mid tones, and then in another colour, the light tones.
Old Billy was a canal horse that was born in 1760 and died in 1822 at the age of 62, being the longest lived horse on record. I drew his scull in yellow, ochre and dark brown pastels.