I think she could be going grey. Horses can be born any color and eventually grey out. Plus she's only 2, so it could just now start happening. If she didn't have a grey parent though, I don't know. Grey is the most dominant color gene (correct me if I'm wrong) so she would have to have a grey parent to be greying out, right? I have no idea about the black.
If neither parent was gray (which it doesn't sound like they are), then there is precisely a 0% chance of her graying out. And, at 2 years old, even if she did have a gray parent, there would be some pretty obvious signs on her face that she was going to.
I don't believe she's graying. What I see is a very attractive brown horse that has some white ticking in her coat. Like others have said, it can be due to rabicano and/or sabino. Some solid horses just get some ticking and, without testing, there is just no way to know what it comes from. She may develop a bit more as she ages...or what she has may fade and become harder to see, but she'll never have enough to look like a "roan" horse.
I have a bay girl marked a lot like this. She has lots of white ticking on body but not her face. There are also a few white hairs on either side of her tail. She gets more white hairs every year and now at 6 years old, she looks almost like a roan. There are some smutty darker areas on her hips and withers. In the winter, her white ticking is much less obvious.