They do in bc.

But really to take that world wide would require pics and stuff. Difficulty is subjective.

Green - Easy as pie. Usually smooth with no steep climbs or descents. Usually a double track

Blue - single track. Not very technical, no stunts, pretty groomed with roots and rocks. This is typically like most xc courses with lung busting climbs. Can have some basic stunts.

Black - single track. Steep technical descents. Usually nothing that can't be walked. But still very difficult for average riders This is usually a typical dh level type run. A true big mountain experience. Can also have some stunts and skinnies that are highish and narrow.

Double Black - Creme de la creme. High skinny stunts, big drops or REALLY steep lines. This requires, nuts, skills and a good bike.

Thats more or less how bc breaks it down.