"Melyik könyvek színesek?"

Translation: Which books are colorful?

[KrisWD]

Why melyik, not melyek, even though könyvet is plural?

[vvsey]

Because adjectives, and question words in an adjective-like role, modifying nouns (in front of nouns) stay singular. Much like in English, actually.

[csarkesz]

The best version of this would actually be "Mely könyvek színesek?".

[Tielbert]

Why? What's the difference?