Turns out, everyone has brown eyes. You can blame melanin. You might have known that melanin determines our hair and skin color, but miniature melanin cells, called melanocytes, are found in the iris as well.
There's really only one 'shade' of melanin - and it's brown. The more melanocytes in your iris, the darker your eye color. Light is also a factor: Melanin absorbs light, so the more melanin there is, the less light will be absorbed. Brown-eyed people have more melanin, less light.