Nought Forever by Malorie Blackman

A fugitive Nought teenager, Dan, crosses paths with Eva, a privileged Cross girl, when he seeks refuge at her home, forcing them to face the entrenched prejudice dividing their world. Over a single, tense encounter, both grapple with secrets, fear, and the risk of helping one another, ultimately confronting whether trust and compassion can overcome suspicion, loyalty, and the danger that shadows them.