This is the 69th most frequent Hungarian word.
milyen
"Milyen" means "what kind of" or "what" in English.
Here, 'milyen' is used as an interrogative adjective to ask about the type or quality of the subject, in this case, 'színű' (color).
Milyen színű az ég ma?
What color is the sky today?
In this example, 'milyen' is functioning as an interrogative adjective within an indirect question, asking about the type of object, in this case, 'könyvet' (book).
Nem tudom, milyen könyvet válasszak.
I don't know what kind of book to choose.
Here, 'milyen' introduces a subordinate clause that describes the quality of the noun 'ez a virág' (this flower), with 'szép' (beautiful) being the quality.