As I painted the more distant hills, I lightened the tone and increased the proportion of blue in the gray paint mix, according to the rules of aerial perspective. I painted the bottom edge of the cliffs a little way below where I intended to paint the horizon. This way I definitely wouldn't have a gap between the top of the sea and the bottom of the hills that I would then have to "fill in" later.
Once the hills were finished, I started painting the rocks in the foreground. The rocks are painted with the same colors as the hills, but with considerably less white in the mix.