When something reddens, its color changes so that it looks red. In autumn, the leaves on some trees redden before they fall to the ground.
If the sky reddens at dusk, you might remember the rhyme "red sky at night, sailor's delight." Pale people redden too, when they blush or flush with embarrassment. A child coloring a picture with crayons might redden the sky and shade the trees blue and purple. The Old English root, meaning "become red," is readian.