Those who are old enough to remember Paul McCartney as a Beatle probably also remember longing for a colour television. (Nowadays, of course, you hardly ever see a black and white one.) There was something about watching television in colour that made even programs like My Mother the Car sparkle. And as for Star Trek–wow!

Human beings have always been fascinated by colour. We use it in artwork, clothing, makeup, houses and cars to communicate, to affect our mood, even to entice prospective mates.

It’s interesting, then, that objectively speaking, there’s nothing special about colours at all: they’re just certain wavelengths of light, and light itself is merely that very narrow band of electromagnetic radiation to which our eyes are sensitive. The phenomenon of colour arises merely because of the way our brains interpret the various wavelengths of light entering our eyes.

Some creatures can see “colours” we can’t. Bees’ eyes, for example, are sensitive to ultraviolet light, which is invisible to us (although there is some indication we would be able to see it if the lenses of our eyes didn’t filter it out). 

Light registers in our eyes on the retina, which contains two different kinds of specialized cells called rods and cones (named for their shapes). Rods give us vision in dim light, but only in shades of gray (which is why we can’t see colour by moonlight, unless it’s exceptionally bright). There are about 125 million rod cells. Then there are seven million cone cells that give us vision in bright light–and our sense of colour.

Cones are divided into three types. One is receptive to red, the longest wavelengths of light; one is receptive to green, the middle wavelengths, and one is receptive to blue, the shortest wavelengths.

Light-sensitive pigments in these cones undergo a constant bleaching and regeneration as they are exposed to these wavelengths of light. This chemical change results in a nerve signal that is sent to the brain for interpretation. The brain takes in the information from all those millions of cells and constructs the image which we see in full, vibrant colour.

But wait a minute. Didn’t I just say that we have cones for only three colours–red, green and blue? Anybody can see that there are more colours than just those three. Where do the yellows and oranges and purples come from?

Well, they come from combinations of the other colours; but this is where things get a little confusing. Everybody who has ever finger-painted in kindergarten knows the colours of paint you mix to get other colours. Blue and yellow give you green, for example, while yellow and red give you orange and blue and red give you purple. But what happens with paints and what happens with pure light are two different things.

With coloured light, only one particular wavelength is being projected and then reflected into your eyes. Additional colours can be created by adding additional wavelengths. Thus, adding red and green gives you yellow, adding blue and red gives you red-blue (magenta), and adding blue and green gives you blue-green (cyan). All colours can be created by varying the proportions of red, green and blue. Add them all together equally and you get white. These three colours are called the additive primaries.

Paint creates colour not by adding various wavelengths of light together but by subtracting from white light the wavelengths that aren’t wanted. The colour you see is only whatever wavelengths were not subtracted. With paint, all other colours can be created by varying the proportions of yellow, cyan (blue-green) and magenta (blue-red). However, if you add all three together equally, you don’t get white, you get black, because then all wavelengths are being subtracted. These three colours are therefore called the subtractive primaries.

Colour televisions and newspaper photographs are good examples of the two kinds of primaries. Colour TVs project red, green and blue dots on a phosphorescent screen in varying intensities. Our eyes blend the light being projected into the appropriate colours. (If you’re only seeing individual dots, you’re sitting much too close.) Newspaper photographs also use dots, but they’re created with pigment–ink–instead of light, so the colours used are yellow, cyan and magenta. Black is usually added, too, to intensify the image.

Photographic film works something like our eyes–it has three pigments that are sensitive to the different primary colours. But we have a little trick cameras can’t handle, called colour constancy. The most familiar example of this is undoubtedly what happens when you take a photograph without a flash in a room lit only by everyday light bulbs. When you get the photo back, everything will appear yellow–yet it didn’t appear yellow to your eyes.

That’s because our brains are able to compensate for changing light conditions to keep our perception of colour constant. Edward Land, the inventor of the Polaroid camera, discovered this in 1955 when he was experimenting with creating full-colour images by projecting red, green and blue segments together. By accident, the green filter fell off one of his projectors–and the full-colour image remained full-colour, without any green in it at all. Later he shut off the blue projector altogether, so he was projecting only the long-wave segment of the photograph with red light and the medium-wavelength part of the photograph with white light–and he still saw green and turquoise and other seemingly impossible colours. His brain knew what the colours were supposed to be and ignored the change in the actual wavelengths being received.

The fact such a complex mechanism exists in our brains is evidence of the importance our species places on colour. No doubt colour recognition is an important survival trait–it helps us identify spoiled food, for example, or to differentiate between poison berries and edible ones–but for most of us, colour is most important for aesthetic reasons.

Or, to put it another way–once we longed for colour TVs; now kids long for neon fashions.

It’s all part of the ongoing quest for colour.

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