I think you have several different issues involved in this.
Legally, to use the term "free range" for your chickens, they just need to have access to the outdoors. They don't even need to go out, they just need to have access.
Your customer's expectations and attitudes are a different matter. I think most people think free range means they're foraging in a field somewhere, loose. Back in the 1800s cattle that were free ranged weren't contained by fences, but I don't think anyone is too disturbed these days by big grassy pastures with a perimeter fence. I think most people expect livestock to be fenced in. I don't think they expect a free ranged chicken to be living the life of feed lot cattle, penned up on dirt. Most people are really shocked and upset, when they find out how commercial birds are raised, that have labels like cage free or free range on them.
I don't know that there is a legal definition for pastured, yet. Certainly the expectation would be that the chickens are living and foraging on green pasture. There is a big difference in pastured and non-pastured chickens and their eggs. I'm not just talking about taste or the way they look, I'm talking about nutrition, too. The types and/or amounts of fat, cholesterol and vitamins are different. You won't find as much validated information on the web about chickens or eggs as you will if you do a search about the difference pasturing makes in livestock meat. You should really check it out. It makes a difference in the amounts and types of fats in the meat. It makes the meat nutritionally superior to feed lot meat. Pastured meat is a big deal. There are small producers specializing in it. Definitely read some of the information that IS out there on the difference in pastured eggs and the tests that have been done. This is something to think about, when you are thinking about how you want to market your eggs.
Some commercial producers have changed what they feed and now label their eggs as omega 3 eggs. It's easy to do, by feeding flax. Eggs from chickens on pasture just naturally have more omega 3 in the eggs. If people are willing to pay for it, I'm sure they'd eventually be willing to feed greens to chickens, to try to further simulate pastured eggs. It's funny how diet makes no difference in the eggs, until it's an advantage for major egg producers.
You might want to think about using a phrase something like, "Access to pasture daily" or "Free range with access to pasture daily" and "Farm fresh" is always good. If it turns out you can't do that, but you want to give them green feed to make the hens and eggs healthier, I'd let the customers know that you provide it.
You might have to just see what your customer base is like. People may want healthier eggs, eggs that are produced less cruelly or you might have some that just want cheap eggs. You'll do better with the first two, as a business.