Eye color is a function of pigmentation both at the back of the iris (iris pigment epithelium)
and in its stroma (the front of the iris), as well as the density of the cells in the
stroma.
In most cases these factors, and hence eye color, are determined by genetics with potentially
as many as 15 different genes identified to date seeming to affect the ultimate coloring
in some way.
However, contrary to what you might think, blue and green pigment do not exist in the
human ocular fluid or iris.
In fact, only a couple of pigments are involved in eye color: melanin (brown) and lipchrome
(yellow).
Aside from potential extreme albinism cases, all eyes have some amount of melanin in the
back of the iris, although not all eyes have lipochrome.
For brown eyes, there is high concentration of melanin in the back of the iris as well
as in its stroma.
The result is the melanin absorbing both short and long wavelengths of light from across
the visible spectrum, with the density of melanin in these two layers ultimately determine
the shade of brown, which in extreme cases can appear near black.
On top of the melanin, amber eyes get their color from the added presence of lipochrome
in the stroma.
As we said earlier, blue and green pigments do not play a role in human eye color.
So how does anyone have blue or green eyes?
Well, the remaining human eye colors, including blue and green, are in part a trick of the
light.
With blue eyes, there is a fair amount of melanin in the back of the iris, but relatively
little melanin within the stroma, making it translucent, with light getting scattered
around when it encounters this layer.
This scattering occurs more at the shorter wavelengths of light, most relevant here being
blue, while longer wavelengths are typically passed through and ultimately absorbed by
the melanin at the back of the iris.
The net result is the eye appearing blue, even though if you were to dissect the eye
in question, you'd see the person physically had brown eyes.
This same type of thing is more or less what's happening in the sky when sunlight is scattered
in the atmosphere, making it appear blue, even though if the light from the Sun was
not being scattered or absorbed in the atmosphere, the sky would look similar to the way it does
at night in the day time.
Grey eyes work almost exactly like blue eyes, with a good amount of melanin at the back
of the iris absorbing the longer wavelengths, but little within the stroma.
The reason these people's eyes don't appear blue is thought to be due to larger deposits
of collagen that cause a different kind of scattering that isn't as frequency dependent.
This type of scattering is basically what is happening when water droplets in clouds
cause them to look some shade of grey.
In contrast to blue and grey, green eyes have far less melanin at the back of the iris,
so slightly less absorption; they also have some lipochrome in the stroma.
As with blue and grey eyes, there is little melanin in the stroma and so a bit of light
scattering helps impart a blue hue, the net result of all of this being the eye appearing
some shade of green.
Hazel eyes, which can shift from brown to green, have more melanin in the anterior (front)
border layer of the eye, and, like green, grey and blue eyes, scatter certain wavelengths
of light resulting in a somewhat shifting eye color.
On that note, the fact that these latter hues are largely dependent on a trick of the eye,
rather than, for instance, blue pigmentation, is why people with these eye colors sometimes
appear to have a different eye color than they normally do.
Nothing has actually changed physically in their eyes, but changing the lighting may
end up changing the wavelengths and concentration of light getting scattered and absorbed, resulting
in the appearance of, for instance, a normally slightly green eye looking slightly blue.
As to why some people have two eye colors, multiple eye colors can be caused by a variety
of things from genetics to various medical conditions, such as Horner's syndrome and
Waardenburg syndrome.
Regardless of the underlying cause, the individual in questions ends up with a non-uniform distribution
of melanin in one or both eyes.
In some cases, the individual might have two completely different colored eyes (complete
heterochromia).
Perhaps the most common cause of this is genetic.
However, other phenomena can cause a single eye to completely change color, such as inflammation
like that suffered by actress Mila Kunis, which changed the color of one of her eyes.
External causes include topical medications used to treat glaucoma, potentially turning
a blue eye to brown.
Another variation of heterochromia occurs where at least one eye displays two different
colors throughout, sometimes with an inner ring differing from the outer coloration (central
heterochromia).
Another type occurs where only a portion of one eye has a different color.
As with more uniform eye coloring, in all cases the resulting eye color is largely a
function of brown and yellow pigmentation and its distribution and density in the back
of the iris and in its stroma.
Speaking of eyes, Warby Parker!
Bonus Facts: Ever wonder why you see random flashes and
the like when you rub your eyes?
Well, wonder no more: Called phosphenes, these are characterized by perceiving some form
of light when there is little or no light actually entering the eye, making it an entopic
phenomenon (meaning the source of the phenomenon is within the eye itself).
Phosphenes are most commonly introduced by simply closing your eyes and rubbing them
or squeezing them shut, tightly; generally the harder you rub or squeeze, the more phosphenes
you'll see.
This pressure stimulates the cells of the retina and, thus, makes your brain think you
are seeing light.
Specifically, Grüsser et al. demonstrated that pressure in the eye results in activation
of retinal ganglion cells in a very similar way to how they activate as a response to
light.
There are also several other ways phosphenes can be generated.
These include through: electrical stimulation; intense magnetic fields; hallucinogenic drugs
(phosphenes not to be confused with hallucinations, which are generated in the brain, not the
eye); a blow to the head or a particularly severe sneeze where extra pressure is created
in the eye on the retina and possible stimulation of the visual cortex; low blood pressure,
which can be experienced if you stand up really fast after you've been relaxing; exposure
to radiation; meditation or extended visual sensory deprivation; various side effects
from medical drugs; and certain diseases of the retina and nerves.
And if you’re wondering, the word "phosphene" was coined by surgeon JBH Savigny deriving
from the Greek "phos", meaning "light", and "phainein", meaning "to show".
Moving on, have you ever wondered what that stuff in your eyes, commonly called “sleep”
or “eye crusties” is when you wake up in the morning?
Well, wonder no more: "Sleep" is a type of "rheum", which is the name for discharge from
your nose, mouth or eyes during sleep.
More specifically, eye rheum is known as "gound".
Gound is made up of a mixture of dust, blood cells, skin cells, etc. mixed with mucus secreted
by the conjunctiva, as well as an oily substance from the meibomian glands.
The meibomian glands are a type of sebaceous gland that line the rim of the eyelids with
about fifty on the top and twenty five on the bottom of each eye.
They secrete an oily substance called meibum that performs a variety of functions including:
helps seal your eyes in an air tight fashion when they are closed; prevents tears from
spilling onto your cheeks; and helps keep tears that coat your eyes from evaporating.
It is this oily substance that is one of the primary components in gound, mixed with mucin
from the conjunctiva and various foreign particles in your eye.
Normally, when you are awake, the gound is naturally washed away via tears and the blinking
motion.
However, as you sleep, you obviously don't blink so the meibomian secretions and other
components of the gound tend to gather in the corners of your eyes, as well as along
your eye lines and dries out, creating hard yellow-ish "eye boogers".