Using a No. 6 Round brush drop in a thicker than water mix of Burnt Sienna + French Ultramarine into each side of the trunk. Try to vary the amounts dropped in so that the overall effect is random. Don't be tempted to start stirring the paint round in the trunk, it will do better on its own.
Add a similar thickness mix of Cadmium Yellow Pale + French Ultramarine in the same way.
Don't wait for it to dry, go straight to the next stage.

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 download a full example from the CD here