As much as I like the feel of a brush, sometimes a knife is called for. For me, this is especially true when two needs coincide. If I need to cover a vast amount of area in similar value or color quickly, and also if I want that color to be "pure," I'll grab my painting knife.
Although the sky is filled with clouds, both cloud and sky are mostly the same value with related colors. The purples of the clouds are very close to the blues of the sky, both on the color wheel and on the value scale. (I really enjoy a cloud when its values are so close to that of the sky that it creates a barely perceptible, soft edge.) To create the different effects, I piled up pure white on my palette and stirred in bits of Ultramarine Blue, Alizarin Crimson and, to warm up the mixture, a speck or two of Cobalt Blue and Sap Green. I didn't add these colors all at once, nor did I mix this pile completely, as I scooped up the mixture and applied it to my panel.
I added Grand Manan, the island on the horizon, while laying in the sky with the same paint and technique. It's got just a little more Ultramarine Blue and Alizarin Crimson in the mixuture.
For the water, again I used the same pile of paint, but I added more Cobalt Blue and Sap Green to make the water warmer than the sky. Enough cool colors -- Ultramarine Blue and Alizarin Crimson -- remained to allow some of the sky and cloud colors to "reflect" in the water. The knife made it possible to make this mix of warm and cool complementary colors without also making "mud."