As of writing this question, there are two questions here on the difference of those two words. The first one has a quite extensive answer and the second one, unfortunately, just a short answer that addresses only the example given there.
I'm quite confident I understood the difference, but I've noticed something that hasn't been explicitly mentioned and therefore I'd like to clarify whether my understanding is correct or not.
Какой seems to be the counterpart to the English interrogative pronoun what and German was.
What is your favorite color?
Was ist deine Lieblingsfarbe?
Какой твой любимый цвет?
Который seems to be the counterpart to the English interrogative pronoun which and German welcher.
Blue or red — Which color is more beautiful?
Blau oder Rot — Welche Farbe ist schöner?
Синий или кра́сный — Который цвет более красивый?
Am I right or would какой also be correct in the second sentence?
A few side notes:
- Который is, of course, also a relative pronoun.
- I'm aware that in a different context the English word what is translated as что.
"What is it?" => "что это?"