Well, yes, it is kind of normal for people's eyes have changed their color when they are in the sun. And according to some researches, it can just depend on how the light reflects off your eyes in the sun. So we can say that sunlight can just reflect you eyes and then in that way, your eye color will be lighter than before. And that is why you look them changeable. Also, you should know that eye color can also change with emotion too, for it is caused by the change in blood pressure. By the way, weather can be another factor of eye color change.
If this phenomenon just happened in sunlight, there is nothing to be worried as far as i am concerned. It is a quite a normal situation owing to the light reflection. When your eyes are in sunlight, a light beam will enter into your eyes, part of the light is absorbed by the eyes, and part of the light is reflected out, cause the retina is just like a minute surface. Therefore, you can see this interesting phenomenon, changing color of your eyes. But in fact, your eye color does not change at all. This is just a kind of optical phenomenon. And you do not need to worry about it. However, if you have some discomfort in your eyes, you must go to hospital for a detail eye check.