The colors also don't tell the whole story.
Both of the courses at the Arizona Biltmore are rated as orange "tough to walk." In fact, both courses are cart only, except for members, which is a ridiculous policy, but not uncommon at Arizona resorts. What's really galling about it is that the Adobe course is as easy a walk as there could be--flat ground, all the holes close to one another. The Links course would be more difficult given hills and hole spacing issues (the nines are quite separate as well).
Not sure a number rating can do anymore to clear up the multitude of ways a course might be "tough" to walk (tough to do the walking vs tough to be allowed to) either, though.