Aww, I've seen toenails before. I even have some of my own.

The brown one looks to me like your basic bruised toenail. I haven't had one of mine turn green but that's one of the colors that bruises sometimes turn while they're clearing up. I guess if one nail got bruised less severely, it might follow a different course of healing.

Here's a Wikipedia article on the condition and here's one from a podiatrist's website.