Europe

.colour systems are used to show the difficulty of a slope.The color system for each country is slightly different - in all countries blue (easy), red (intermediate) and black (expert) are being used. Shapes are not always used. The ratings are:

Green
(Spain, France, Scandinavia, UK, Poland) Learning or "Beginner" slopes. These are usually not marked trails, but tend to be large, open, gently sloping areas at the base of the ski area or traverse paths between the main trails.
Blue
An easy trail, similar to the North American Green Circle, and are almost always groomed, or on so shallow a slope as not to need it. The slope gradient shall not exceed 25% except for short wide sections with a higher gradient.[2]
Red
An intermediate slope. Steeper, or narrower than a blue slope, these are usually groomed, unless the narrowness of the trail prohibits it. The slope gradient shall not exceed 40% except for short wide sections with a higher gradient[2]
Black
An expert slope. Steep, may or may not be groomed, or may be groomed for moguls. "Black" can be a very wide classification, ranging from a slope marginally more difficult than a "Red" to very steep avalanche chutes like the infamous Couloirs of Courchevel. France tends to have a higher limit between red and black.
Double-black
(Scandinavia) Extremely difficult.



Yellow
Extremely dificult(Austria, Switzerland, certain other areas)
In recent years, many resorts reclassified some black slopes to yellow slopes. This signifies a skiroute, an ungroomed and unpatrolled slope which is actually off-piste skiing in a marked area. Famous examples are the Stockhorn area in Zermatt and the Tortin slopes in Verbier. In Austria, skiroutes are usually marked with orange squares instead

(ratings from wikepida)