On a sunny day the light of the sun is warm and bright and makes colors warmer and more intense. The light of the sun does not make things only lighter, it makes them yellower too. So to show that a patch of grass is in sunlight, the color needs to be lighter and warmer. On the same note, if the sunlight is red or blueish, every object in the scene will have some of that color reflected on it. In overcast days all colors are duller and shifts in value are less noticeable.
