When we underpaint a scene, consideration needs to be given as to the colours used on top.  For example, in the painting above, we know that the yellows/oranges/reds of the sky can easily take and allow the strong colours of the blues and greens to be placed on top, thereby creating a contrast.  There always needs to be a contrast, ie light against the dark to create the wow factor.  If a painting was all in mid-tone colours, there would be nothing to catch the eye.  We then pull down a little yellow and red that was in the sky to the foreground to create balance