I am afraid that neither of your options seem completely right, and "av" is in any case not the right preposition.
It would be easier to answer if you could say more about your context: what do you mean with "colour of sunset"? Are you describing a specific colour that is often seen in sunsets? Or are you looking at a specific sunset, and talk about the colour(s) that you see there?
I think it would be useful if you could give us a complete sentence (in English) that includes "colour of sunset".