Mike Rooney, the master I am currently following, sketches in magenta. I've already tried to rebel against this, but it's by far the best sketch color to use on the canvas... warmth flows through all the colors you put on the final surfaces. Guru Mike was right - magenta is the way.
The next step is the first phase: the underpainting. Everything dark is deep purple. Everything in the lightest tones is yellow. Second darkest is blue. Second lightest is orange. Third darkest is green. In this instance, I've gone to grays for the third lightest... the white Michigan sand has a definite gray undertone, and I'm thinking this will help bring that out.