The daily sketches of Guy Denning. Particularly like the centralisation of the image here. Note: Play around with contrast and how darker colours don't need to be confined to the centre of the page, they can be inverted.
Inktense watercolour pencils on a Strathmore, grey-paper sketchbook. For tips on how to use watercolour pencils see: http://lynnechapman.blogspot.co.uk/2012/10/how-to-draw-with-watercolour-pencils-10.html