Three layer painting.
Layer 1. Local Colour
The first layer consists of a wash showing the 'local colour'. Local colour is the colour you see before you - the basic colour of an object. So the local colour of an apple might be red, and of the sky, blue.
In my painting here I have blended lots of different local colours wet-in-wet to show the rocks and trees that were in front of me. Don't concern yourself with any details at this stage. Just put down the local colours as quickly as you can, using the largest brush you can get away with.
Click on the picture for a larger version.
Layer 2. Add Shadows and Darks
As soon as the first layer of paint is dry you can come in with the shadows. Use either a purple mix (ultramarine and cadmium red makes a fine shadow colour) or use a darker form of the local colour. In this painting I used both. Again, don't fiddle with too much detail at this point.
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Layer 3. Add the Details
Finally, when the second layer is dry, you can come in and add details, possibly using a smaller brush if you need to. Add details sparingly, and stop when you can tell what the painting is. You can leave quite a lot to the viewers imagination. Here I just added a few dark lines for tree trunks, and fussed with the shadows a bit more.
The Finished Sketch
So here's the finished sketch, completed in less than 15 minutes. Click on the picture to see it in a larger version.
This sketch may be too loose for your liking, but you could also do a detailed line drawing in your sketchbook as well so that you would have both accurate detail and colour notes for a painting back at home.
Here's what I was painting
Here was the scene in front of me - a rocky outcrop on the edge of Derwentwater in the lake District.