Green is my favorite color, so unless it's a drab green or it floureses, I think it would look fabulous. And on the plus side...it should blend in quite nicely with your local flora. On the down side...it could look as if you just had some leftover green paint...lol !!!
I'm hoping to put a green roof on my coop. Hubby is building it so we'll see if he goes along with that. We are in Canada though so I'm not sure what plants or grass will survive winter on the roof. I'd rather not replant every spring. Planning to plant herbs that can be used in coop like oregano etc as well as grass.
Moss is a good idea too. I don't think sedum would winter well for me. The oregano and thyme in my garden spread like crazy and never die in winter so I'm hoping that will work. Might just use sod that we dig up under the coop footprint to start with then plant different things in there over time.
This is my herb garden above a portion of my run. Roughly 5" deep, made out of old treated deck boards, lined the entire thing with heavy plastic to keep the wood from rotting, but left a good opening for drainage on the low side.