Whats the name of this book about these kids living in a city with colored ditricts and runaway?

this book is fictional but these kids live in a city that have different districts that have different colors and the different colors show rather u have money or how much but these kids climbed up this big statue in the middle of the city and everyone noticed and they ran away through the sewers where they met some people who lived there and grew something in the water some kind of food and they left there. i think there characters were brother and sister