The light of the sun is scattered by dust or vapour in the atmosphere. The light is also bent when the sun is close to the horizon, and greatly distorted. This analysis shows how much light there is in the darkness around the sun. See how the red and green values do not drop immediately to zero, beyond the sun's disc.

Here the edges of the mountains are given extra emphasis, or contrast, by dropping the dominant colour (red in this case) and boosting the blue and green.

This aurora picture is a good example of light in the sky against a dark background. The red, green and blue values seem to follow each other, although blue has another component from the background of the dark sky.

Picking skin tones is a tricky job. Here is the Pope, and a vertical analysis showing the range of colours from forehead to chin.