Should be 4 categories Easy, Moderate, Hard, Extreme.
This
Green
Blue
Black
Anything beyond that and people will lose interest and therefore the signage will become less affective. It's also more cost affective.
Or This. Keep simple and strait forward. Nothing more than this needed IMO. An Easy trail is just that, 'passable by most people. Loose terrain, small rocks or hard-pack. 4wd recommended but not typically required.' Groomed bunny slop.
Moderate: 'You might have to work to complete it'. Most people can figure that out.
Hard: 'Your going to have to work harder and have a better built rig. Damage and/or flop/roll very possible'.
Extreme: 'It ain't easy. And you better be prepared. Purpose built rig territory'.
The Green/Blue/Black or Green Circle, Blue Rectangle, Black Diamond (and Double Diamond) if VERY recognizable and pretty universal from Skiing/Snowboarding and many other outdoor sports. Probably the earnestly recognised and would be easy (and cheap) to procure as they have been pre-made for Ski sloped for decades. Many of them even say "Easy, Moderate, Difficult and Most Difficult" on them. Two birds with one stone.
Like Brad said, make it much more complicated beyond that and you loose people, plus it makes it harder for those assigning the designations to figure out.