“With the development of landscape painting, there came new ways of seeing. Instead of always painting from the back to the front (such as sky, mountains, and then trees) the artist began to see that many times the backlight popping through and around an object forms the object.”
Stephen Quiller, “Color Choices”, p111
Perhaps the easiest example of this is painting blue dashes of sky over the greens of tree leaves, rather than leaving gaps between leaves to let blue from a background show through.