Why did my flank steak turn grey when I brined it?
It is highly likely that the acid from the cider vinegar denatured the proteins at the surface of the meat, making them opaque instead of translucent, so the red interior does not show through.

This is akin to when ceviche is "cooked" in citrus juice, turning opaque.

This cannot be reversed, but when you cook the steak and browning reactions take place, you should get a better appearance.

In general, you can prevent this while brining by not including acid or enzymatically active ingredients like uncooked pineapple juice.   Brines in general should only contain water, salt, and sugar; other flavorings are common, but they have little to no effect as they do not penetrate.