Sounds like the chicken injured itself on something in the run or while out foraging.
The color green is common for bruising.
Gangrene results from an injury that has poor circulation. It oozes and smells really bad. You'll know if that is the problem.
Keep the chickens wounds clean, apply Vetricyn Spray for poultry, if you don't have that, Neosporin triple antibiotic ointment will do. Just make sure there is
no pain reliever in the ointment. Any pain reliever ending in "caine" is toxic to chickens. Attend to the wounds twice daily.
It's good you isolated the chicken, but remember, watch for depression, it may need some supervised time outside during its recovery. It's best not to return it to the flock until the wounds have healed substantially. The other chickens may peck at them, and probably already have.
As long as the chicken is eating and drinking well, it should heal up just fine.
Try and determine what injured the chicken, and rectify the problem. Also, sometimes we think the injury came from an object, but instead, it was a predator. It is not uncommon for young chickens to be bitten by rats or possums, that get into the run or coop. There have even been cases of hens getting eaten alive by a large rat, while laying an egg! There are times when a chicken is very helpless, like during sleep and while laying an egg.
Also, if you are interested in the natural healing of wounds with Manuka honey here is a fascinating post about a rooster that was critically injured by a coyote. Then, his wounds treated with the Manuka honey.
https://www.backyardchickens.com/threads/rooster-rescued-from-a-coyote-using-manuka-honey.1202992/
These are my thoughts, I hope they are helpful.
God bless.