If you are able to watch DVDs and take screencaps from your computer, take a picture (preferably a closeup) of Legolas from one of the scenes from the Council of Elrond. If you look carefully, you'll notice that Legolas's eyes are blue. The colors of the background make his eyes look brown.
I've noticed this a few times as well- like in the trailer for two towers, right at the end Legolas's eyes go REALLY icey blue and it is a close-up shot of his eyes. But normally they are brown. The only possible solution i can think of (and i have to admit i haven't read the books so i dont have as much knowledge) is that his eyes change colour to do with his mood, who he is with, where he is, what he is thinking about...etc.