I felt the need to research/summarize the following information about my eye color in order to post this picture:
In humans, the coloration of the iris varies depending on the concentration of melanin in the iris pigment epithelium (located on the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma.
A blue iris in an eye is due to Tyndall scattering/the Tyndall effect in a translucent layer in the iris called the stroma. The fiber structure of the stroma scatters light, with the tendency to scatter short wavelength light more frequently than long wavelength. Brown irises have more melanin within this layer which absorbs light. In blue eyes, however, there is no melanin present in the stroma, causing the layer to be translucent/essentially colorless until light enters and is then scattered and reflected back into space. The light passing through is randomly and diffusely scattered, causing an observable portion of the light that enters the stroma to re-emerge via a scattered path. This redirection of the lightwaves back out to the open air is called backscatter, and scattering takes place to a greater extent at shorter wavelengths. The longer wavelengths tend to pass straight through the translucent layer with unaltered paths, and then encounter the next layer further back in the iris, which is a light absorber. Thus, the longer wavelengths are not reflected (by scattering) back to the open air as much as the shorter wavelengths, or the blue wavelengths, are. The result is a blue hue in the light that comes out of the eye; this is why the color of blue and green eyes depends on the quality and quantity of light available in a room. The term for this phenomenon- of material appearing to have a certain color while no pigmentation of that color is present- is called structural coloration; a blue iris is an example of a structural color, in contradistinction to a pigment color.