In an undefined future world, humans have lost the ability to see all parts of the color spectrum. Color vision is inherited; some folks can see red; some yellow; some blue, etc. A caste system has been created where those who can see color are arrayed above the Greys, who can see no colors. There are also dangerous beasts and human-eating plants around the countryside, so everyone must stay inside carefully defined habitation areas. Plus, it's dark at night. Not being able to see the full spectrum means eyes that can't see at all in the dark.
It's a very different sort of book from the Thursday Next novels, full of a certain disturbing menace and confusing plot twists, and a government that passes and enforces ridiculous rules like Leapbacks, where certain technologies become illegal, and a ban on spoon production.