Rendering exposed wood grain is one of those cool effects that is rather easy to achieve. Using pastel colors from darks such as burnt umber and burnt sienna to burnt orange and ochers I apply them to the area in uneven layers, so some places will be darker and others lighter. Then, with my fingers, I blend them together in the direction of the grain. Careful not to overdo it, I don't want to end up with a uniform color. Then take a charcoal pencil and draw in lines of wood grain, knots and the grooves between the planks and viola!