Whenever there are Tom Hiddleston fans, there is always confusion about the man's real eye color, and whether or not he was wearing contact lenses. Are his eyes blue or green? Was he wearing contact lenses part of the time?
As near as I can tell, Tom Hiddleston's eyes are most probably blue-green. Depending on the ambient lighting, blue-green eyes can appear blue or green - and if you pay attention to the photos, they always look green when there's a yellow tint and blue when there's a bluish tint.
To demonstrate how this works, I took some photos of my own eye so you can see just what ambient lighting can do to eye colors like that. Observe:
The first picture is taken in cool light, which makes the iris look blue - even almost gray. The second picture is taken in neutral light, which makes it pretty close to the actual color. The third picture is taken in warm light, so it appears green.
Remember when Loki's eyes looked green? That was when he was in the palace of Asgard, which had a golden cast to it. It probably wasn't that they "forgot" to give him contact lenses in other scenes; most likely it just the differences in lighting.
So whenever you're trying to determine someone's eye color in the future, remember to take ambient lighting into account.
Addendum: I have been told that at some point Tom Hiddleston actually said something about the color of the eyes being manipulated. However, I have never been able to locate the exact quote. My suspicion is that if he did indeed say this, he may have been talking about the promotional poster with Loki, which definitely were manipulated.