Here I have added some Permanent Sap Green to the tree trunk. After it dried I used some Burnt Sienna and created a wet stripe down the tree trunk. Before the pigment dried I softened both edges with a clean, damp brush. This became a soft shadow on the trunk.
If you would like to get the drawing and photo references for this project, please visit my online etsy shop and click on the Online Class Icon. For $5.00 I will send you a pdf file with all of the references and the drawing so you can follow along. Most people prefer to order the references, and print the daily lessons until they are complete. Then they use the printouts as a workbook to do the painting.