What is this book called?!?

[melei]

I read a very interesting book about five or six years ago. For the life of me, I can not remember the name of it, and I have lost my original copy. The cover art was a picture of pine trees from far away, with maybe a valley in the background? It was mainly green. The plot was about some child who was part of a city that had a social class categorized by colors. I believe he was in the grey class, the lowest, while there were higher up classes including orange, white, and others. I can barely remember any details besides him going on an adventure with a few of his other friends into some far away place. The book had a sequel, with the word "slaves" in it. If anyone remembers any books like these, I would greatly be appreciated. Thank you!

[NickeeCoco]

I haven't read it, but I believe you're talking about William Nicholson's Wind on Fire trilogy. At least, what you described fits the bill and the second novel is called Slaves of the Mastery.

http://en.wikipedia.org/wiki/Wind_On_Fire