What is a green trail? In the dessert, that's an oxymoron. I'm familiar with real trails that go some place. I've also seen so called rock crawling trails in the east that are built by a land owner who has boulders placed on a hilly landscape to have people pay a fee to destroy their own truck. The trails that go somewhere can be negotiated in EBs, CJs or FJs with nothing more than wear on the suspension and drive chain. In fact a competent and prudent driver can get into and out of spectacular places with little more than some surface scratches that can be buffed out.
Green trails are "easy" trails. Different parts of the country have different rating systems. Some of the OHV parks in the eastern US use a system similar to ski resorts: Green, Blue, Black with Red even more difficult than Black. Moab uses a 1 to 10 scale and most of the south west uses a 1-5 scale (some trails get a 6 as they get harder and harder!). There are probably other rating systems as well since its not standardized at all. Here is the scale LEBC uses at K2 (Greens would equate to 1's and easy 2's):
Level 1 Trails - Suitable for stock vehicles. Seat belts for all occupants required. Battery hold down required. Hard top or roll bar required. Paint scratches possible, body damage unlikely.
Level 2 Trails - Generally suitable for stock or slightly modified vehicles. Level 1 requirements plus front and rear recovery hooks. Recommended larger than stock all-terrain tires, recommended rear traction device (limited slip, lunch box locker). Paint scratches likely (depends on trail) and minimal body damage possible.
Level 3 Trails - NOT suitable for stock vehicles. Levels 1 and 2 requirements plus fire extinguisher. Recommended front winch, front and rear lockers, minimum tire size of 33 inches. Front and family cage recommended. Body damage possible.
Level 4 Trails - Levels 1-3 requirements plus front and rear lockers, front winch, front cage required. Recommended minimum tire size 35 inches. 5-point harness recommended. Requires previous DRIVER experience on Level 1, 2 and 3 trails. Body damage probable.
Level 5 + Trails - You will already know if your vehicle meets the requirements for these trails! (And you probably don't have much of a body - what's left is already scratched and dented!)