Quick sketch of Loch Linnhe - rain again. This was done in my moleskine watercolour sketchbook.
 I'm always in a quandary as how to render the pale bark of the birch trees, painting around the trunks is difficult and time consuming when sketching, using masking fluid also requires time for the stuff to dry. Wax resist might work but recently I have either been adding acrylic back a home or as here using distress markers. The picket fence white will produce an off white when placed over washes. When you use the pen it takes a few seconds for the whiteness to develop so at first you cannot tell where you've drawn.I also like some of the other colours- they are water soluble so you can blend them and the colours are muted so  not too insistent. In the sketch above I added some of the dark tones in the sky with the beige distress marker. I'm sure its cheating but when sketching I'm always looking for short cuts to apply colour or tone quickly. I sometimes use derwent graphtint pencils  dry, then blend with a brush and water.The problem with these pens and pencils is that they lead to sketch kit inflation - not so good when you are running with a rucksack. A small watercolour set is still the most versatile, and lightweight way of rendering colour sketches.