The government is collecting the children/adolescents who survive into camps, where they sort them into a color by ability. Although Ruby is an Orange, she sees how they are treated and convinces her tester to sort her into Green, which is a class treated better. Greens are intellectuals and have abilities related to their intelligence, while Oranges can change thoughts/feelings and alter other people's minds. Other classes have different abilities. Ruby is interned at Thurmond, which is one of the most well-known and harshest camps in the US. Life is harsh there, and so when Ruby has a way out (is practically forced into it), she takes it.
However, the world has changed in the time Ruby has been at Thurmond, and she must learn quickly to sort friend from foe if she wants to survive. Fast-paced and with some really great characters, this was overall a pretty good YA dystopian, and I am definitely curious to see how this will continue in future books!