Watercolor Sky

What is a sunset but just a watercolor sky?
Not pencil-sketched shapes of clouds against the noonday blue;
Not cutout reflections pasted on a pond with glue;
Not an earthly mural painted with an oily hue,
But pigments thinned with sunshine, moistened with clouds so high,
Washes which upwards towards transparency accrue,
Tints changing on a whim with each moments new view,
Until too few rays are left for colors to imbue;
And the artist folds his palette and moves on; but why?