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. 