Did you think that blue eyes are blue because they contain blue pigmented cells? Did you think that green eyes are green for the same reason?
That colorful circle around your pupil is the iris. The iris is made up of two layers of cells: the front layer is known as the stroma, and the back layer is known as the epithelium.
The epithelium is a layer with a thickness of two cells and containing dark black-brown pigments. The little specks and strings of black you see in the iris? — that’s the epithelium.
The stroma is made up of colorless collagen fibers. The stroma only occasionally contains brown melanin pigmentation. Sometimes the stroma is totally clear, containing no melanin.
The stroma is a delicate interlacement of fibres. Some circle the circumference of the iris and the majority radiate toward the pupil. Blood vessels and nerves intersperse this mesh. — Wikipedia
In brown eyes, the stroma contains a high concentration of melanin, which absorbs most light and creates a dark brown hue. In the illustrations below, “collagen” refers to excess collagen deposits in the stroma.
In hazel eyes, the stroma contains a moderate amount of melanin, giving a light brown hue with occasional sections of green and yellow. Because of the moderate amount of melanin, a good portion of light is scattered back into the atmosphere by the stroma.
In green eyes, the stroma contains a small amount of melanin which provides a light brown hue that mixes with… well, what exactly does that brown light mix with to create green light?
The answer: blue light in the eye is generated by a phenomenon similar to the one that turns the sky blue. The Tyndall effect is the scattering of light by tiny particles floating in a liquid solution. The fiber structure of the stroma scatters light in a similar way, tending to scatter short wavelength light more than long wavelength light.
So, in the case of green, the eye contains no blue pigments but instead scatters incoming light back out into space along with the brown melanin, which creates the green hue.
In blue eyes, there is no melanin present in the stroma. The stroma is essentially colorless until light enters and is then scattered and reflected back into space. This is why the color of blue and green eyes depends on the quality and quantity of light available in a room. This phenomenon of material appearing to have a certain color while no pigmentation of that color is present is called structural coloration, and it’s awesome. It occurs in beef, berries, butterflies, and more.
Grey eyes are a curious case, and extremely rare. Grey eyes are similar to blue in that they contain no melanin, but they do contain excess collagen deposits in the stroma that interfere with Tyndall scattering, blocking the appearance of blue hues. (That’s the current theory anyway).
Given that Tyndall’s bias toward blue light is blocked, all wavelengths of light entering the iris are scattered and reflected equally, creating an even grey hue.
Imagine that you could shrink yourself to a microscopic size and then climb through the mesh of fibers in the stroma. That’s where structural coloration is coming from…
… and in the mesh are also strands of smooth muscle tissue that contract to dilate (expand) the pupil, pulling the inner edge of the iris toward the outer edge. When this happens, the stroma fibers slacken and may become wiggly as tension is released. This makes me wonder, does that slightly alter the color of your eye as well?
UPDATE: To everyone curious about Elizabeth Taylor’s mythical violet eyes, the short answer is that — as far as I know — she had grey-blue eyes that could be made to appear violet with the appropriate lighting or color editing of images.