Now I block in the colors of the sky. I mix one part ultramarine blue to three parts white and just a dab of phthalo blue. When painting the sky, I ?cut in? around the palm leaves, meaning that I create the shapes of the leaves by painting the sky around and between them. (This technique is called ?negative painting,? because you define an object by painting the negative space around it rather than painting the object itself.) Next I add more variation to the color of the water using light blue mixtures of phthalo green and phthalo blue. For the shallowest water, I use white mixed with a dab of dioxazine purple and Indian yellow. Then I add a bit more yellow and orange to the white and develop the highlights on the sand. I paint the dark areas of the palm leaves with sap green mixed with alizarin crimson and allow the painting to dry before I continue to the next stage.