How are European and Japanese runs graded?

The way the resort grades them. A resort in the US or Canada uses green circle, blue square, black diamond and double black. A European resort uses blue, red and black pistes, green for a beginner slope where one is marked, and itineraries and freeride routes as their own group. A Japanese resort uses green, red and black. OpenStreetMap records how hard a run is, not its colour, so an "easy" run is a green circle in Colorado and a blue piste in Zermatt. The Runs step, the trail signs, the trail colours and the printed inks all follow the resort's own scale.