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I read this book about 10 years ago, in French, but I don't know if it was a translation or the original language.
The story takes place in a big city, where each district is painted in a different colour. And the colour depend of the wealth of the people living in it. For example, the poorest peoples lives in the Grey district, in small appartments. But richest peoples lives in the Yellow district (I am not really sure of this colour...), in big villas.
About the story in itself, I remember that we were following a teenager (male I think but not sure), from the Grey district. He ends up going into the Yellow district and encounter another teenager (female I think but not sure). At some point they had to ran away, and went in a large underground passage, and maybe went out of the city. And maybe they encountered a big monster at some point, but I can't tell for sure.
Anyone know about this book?