downhill skiing is a sport. skiers ski down a trail (also called a "run") on the side of a mountain or hill. each trail is marked with a sign that shows how difficult that particular run will be. three different colors are used to rate the difficulty of the trail. they are: green, blue, and black. green trails are easiest, blue trails are more difficult, and black trails are rated most difficult. the colors which represent a trail will also be seen in a shape.