Wood is a natural product, which can change color over time. Some woods have more obvious color change than others.
Tulip Poplar will retain any color contrast. However, all the green and purple shades will fade to a warm attractive brown. The lighter, white looking, coloration will become more cream color.
Wood will bleach if left in direct sun; the temperature difference from the sunny side to the shaded side of the item can also have detrimental effects.