The range of Siamese eye colors. From left to right: whitish blue, medium sapphire blue, and navy blue.

Siamese Eye Colors from Whitish Blue to Navy Blue

By Cris Bird, Dr.P.H.

The following article was originally published in the September/October 2001 issue of Journal of the Old-Style Siamese, the bimonthly newsletter of PREOSSIA.

Did you ever wonder what the sky is made of?

It’s important. When you understand what the sky is made of, you can start to understand why the sky is blue, as well as why it is pale blue at some times, deep blue at others. And if you understand what makes the sky look the way it does, then you can understand what makes the eyes of Siamese cats look the way they do.

Far beyond our planet is deep space, which is mostly vacuum, nothingness. But as you go from deep space toward Earth, you encounter the atmosphere, a blanket of tiny air molecules, as well as tiny dust particles and ice crystals. The atmosphere, as seen by human eyes from down on Earth, is the sky. You could say that the sky is a whole lot of very fine, tiny, breathable particles spread thinly in a belt between the earth and outer space.

When sunlight streams through the sky, it is a mixture of many different colors of light. Light in colors such as red and orange travels in long, stretched-out waves. It has no trouble passing between the millions of tiny particles that make up the sky. It slides between and around them in much the same way that a piece of thread goes through the eye of a needle. Picture millions of red threads of light piercing the sky without a hitch. During most times of day, red and orange and yellow light waves, even green light waves, are gone from the sky before you can blink. You don’t see those colors. However, blue light travels in short, tightly coiled waves. It’s somewhat like frizzed-up thread refusing to go through the eyes of a multitude of airy needles. Instead of sliding right past, blue light tends to hit all the ittty bitty particles in the sky and bounce around up there for ages. Consequently, when you look at the sky during the day, usually you see lots of blue light.

The color of the sky can change. Obviously, if a great deal of material such as smoke pours into the sky, all we will see is the smoke. What is more interesting is what happens when tiny amounts of particles are added to the sky. Add a trace of smog or thinly spread-out water vapor, and the sky becomes a paler, duller blue. Add just a little more water vapor or pollution, and the sky soon appears almost white.

The crowding of additional fine particles together in the sky tightens the spaces through which light waves can pass. Now, not only threads of blue light but also threads of light of all different wavelengths bounce back from those airy needles. When all wavelengths are represented, light looks white to the human eye. When extra particles are added to the sky, it is like adding white light to the blue light already present.

The eyes of Siamese cats are like the sky. Their eyes contain only traces of tiny pigment particles. Most light passes through, but blue light remains in the eyes for a while, bouncing off the fine particles. Siamese eyes come in a variety of different shades of blue. They can be so pale as to appear almost white with just a touch of blue, and they can be so dark as to appear almost navy blue. In between you can find every shade of blue from light sky blue to sapphire to blazing cobalt blue. The different hues of Siamese blue eyes exist because of differing amounts of fine particles present.

That’s right. Some Siamese cats have just a tiny bit more pigment in their eyes than others. The cats with slightly more pigment in their eyes have lighter blue eyes. If they have enough pigment particles present, their eyes can look whitish blue. The pigment is not white. There isn’t enough pigment present in Siamese eyes to impart a color due to the pigment itself. However, the tiny traces of pigment that are present serve as additional fine particles able to bounce light in longer wavelengths. The more particles of pigment are present, the lighter the blue color of Siamese eyes. The more sparse the particles of pigment are, the deeper and darker the blue color of the eyes.

The color we see in Siamese eyes is colored light. It is not colored pigment. Some Siamese merely have a higher ratio of white to blue light bouncing in their eyes than other Siamese.

All cats inherit genes that in the absence of the Siamese mutation can produce green, yellow, gold, or copper eyes. The genes produce those colors by increasing or decreasing production of melanin (pigment) in the eyes. Green-eyed cats produce considerably more melanin than blue-eyed cats. In fact, green-eyed cats produce so much melanin that the green color of the eyes is directly due to the color of the melanin itself. However, green-eyed cats produce less melanin in their eyes than yellow-eyed cats. Yellow-eyed cats produce less melanin than gold-eyed cats. Copper-eyed cats produce the most melanin of all. Copper is the color you see when you are looking directly at a lot of melanin in a cat’s eye.

Siamese cats, like other cats, inherit the genes for producing a certain amount of melanin in the eyes. Because they also inherit the Siamese mutation, their melanin-producing enzymes are not functional enough at warm eye temperatures to achieve rich green to copper eye colors. The eyes of all Siamese cats end up with only tiny traces of melanin; like a clear sky, the eyes are suffused by blue light instead of being colored by pigment. Siamese eyes therefore look blue.

But it turns out that the Siamese cats carrying hidden genes for copper eyes will produce melanin in larger trace amounts than Siamese cats carrying hidden genes for green eyes. The Siamese cats with hidden “copper genes” will therefore scatter more white light in their eyes. Their eyes will look very pale, whitish blue. Siamese with hidden “green genes” will have very deep, dark blue eyes. As you may have guessed, Siamese cats with the right mix of hidden genes for eye colors such as chartreuse, yellow, or gold will end up with blue eyes that are intermediate in color between whitish blue and navy blue.

Now look at the full spectrum of possible Siamese blue eye colors in the side panel photo accompanying this article. Picture in your mind’s eye another spectrum from green to yellow to gold to copper. That green-to-copper spectrum of the domestic cat underlies the whitish blue-to-navy blue spectrum of the purebred Siamese cat. The same genes are behind both spectra. The only difference is that Siamese cats also inherit a Siamese mutation. The Siamese mutation greatly reduces the amount of pigment produced in the eyes, allowing light to fill the eyes instead, and that shifts the eye color spectrum from green/copper to whitish blue/navy blue.