Note when the sun sets the top of the sky retains some of it's blueish elements (albeit more warm and less saturated). The sun, however, becomes very orange and red around its adjacent area. These warm red and orange colours permeate the environment and the highlights of the clouds. The clouds, in this example, are relatively opaque and maintain a strong contrast against the sky by not letting much light through.

At the sun hits the horizon everything in the landscape, bar reflective surfaces, becomes black. The sky remains for a brief time highly saturated. Not long after, though, the sky becomes much more desaturated