All human eyes are brown. This is not a drill.
According to a piece by CNN, regardless of whether you have hazel, green, blue, or brown eyes, the fact is that they are all varying shades of the same color. To be fair, though, it depends on what you define as “color”.
Melanin, comprised of melanocyte cells, is a dark brown pigment. All your irises contain this pigment, which means that, technically speaking, all eyes are brown to some degree.
The more of this pigment you have inside the iris, the more visible light is absorbed, which means that the iris appears to be browner. With a little less melanin, less wavelengths of light are absorbed and greener colors are scattered back.
Those with the lowest concentrations of melanin have blue eyes for the same reason. In fact, plenty of babies are born with blue-looking eyes, because they have not necessarily developed all their irises’ melanin yet.
Fair enough, we suppose, but it’s a little pedantic to say that because of this, all eyes are merely brown. After all, a similar mechanism of color manipulation is used in the plumages of plenty of birds.
Although some birds use pigments to produce a color, many manipulate their appearance by altering the nanostructures of their feathers. By changing the size of these nearly invisible holes, different wavelengths of light are scattered.
Smaller holes produce a bluer effect, while larger gaps scatter larger wavelengths and can appear red. The thing is, you wouldn’t refer to these birds as “appearing blue” in the same way you wouldn’t go up to someone and compliment them on their “apparently green but actually brown” eyes.
So although it is indeed true that all irises are brown, we wouldn’t worry too much about declaring all eyes as being varying colors of chestnut.