Blue eyes are odd that way -- it is totally dependent on the lighting. On an overcast day, my eyes are greyish blue. On a sunny day, they are much more blue, almost a cold sky blue. My dad's eyes are literally the color of a robin's egg, and only rarely do they take on a gray cast. Three of my four blue-eyed brothers have much darker blue eyes that (depending on the lighting and weather) can range from nearly navy (which looks close to brown in a photo) to a medium-dark blue; the other brother has blue eys that are closer to aqua in color and never look grey, even in overcast weather. I have a sister in law whose eyes are practically turquoise, and no matter the weather or what color she is wearing, they ALWAYS look turquoise.
It's just the nature of blue eyes.
That’s because blue is not really a color for the eyes, it’s an absence of pigment. The structure of the iris causes alterations on light refraction, not too different of why sky looks blue. It’s called “Tyndall scattering” (https://secure.wikimedia.org/wikipedia/en/wiki/Tyndall_effect).