It appears that he has pre-toned the canvas in the lower area with a thin layer of burnt sienna which was dry when he arrived the location. The gray area of the beach at lower left and the blue colors of the sea at right are laid opaquely over the warm underpainting.
Those blue colors are just a flat base coat that, had there been time to finish, would have been detailed with waves. The top area of the rocks are subdivided further and further with the brush, progressing from large forms to small ones.
|William Hart, White Pine, Shokan, Ulster County, New York, c. 1850-'60|
Watercolor from Albany Institute
More insights on Hart on Mike Ettner's blog