Tonight we're going to think about colour: what it is, where it comes from, and what it means.

Even if we know nothing about the first two-what colour is and where it comes from-we all have ideas about what it means.

[Pick some people and ask them why they like to wear what they're wearing. See if anyone is wearing yellow, and if not, ask if anyone would (and why not!)]

In China a few hundred you wouldn't have been wearing yellow either, but for a very different reason: you would be risking a death sentence. Only the Chinese emperor was allowed to wear yellow, right up to the 20th century. The word for yellow and the word for emperor-huang-are the same in Mandarin Chinese.

The same was true of purple in ancient Rome. During the reigns of some of the Roman emperors, no one but the emperor himself could wear a purple robe. It was sometimes permissible for generals and government officials to wear little bits of purple-bands at the borders of their togas, for example-and this showed that they were someone very important. But if you wore more purple than your rank allowed, you were in serious trouble. Anyone other than the Imperial dye works who tried to make and colour cloth with a pruple dye was liable to be executed.

So if you think it is important what colours you wear now, it was once a matter of life and death. Colour was used to send out a very clear and important message about one's status in society. Why purple, though? Well, I'll tell you shortly.

Colour is a very immediate way of telling other people who we are and what we think. For example: [pic of football supporters] We don't know anything about who these boys are, but we know right away that they support Man United. And of course, they have chosen their colours to match those of the team strip itself [pic]. On the football pitch, colour helps the players distinguish their side from their opponents.

Well, this is all part of an old tradition. [pic of foot soldiers from the Napoleonic era.] Today, soldiers tend to wear camouflage in order not to be seen, but in earlier times warfare was conducted in a very different way, with armies marching to face one another in massed ranks, and the main thing was to distinguish friend from foe. These colours might also be chosen for other reasons. The British redcoats, for instance, relied on a reputation as staunch and well disciplined fighters, which they thought would be enhanced if they looked splendid. It is also said that the red coats were good for morale because they helped to hide the blood of the wounded.

Even longer ago than this, colour use in armies was more complicated. [Heraldry pic] Knights used colour to say a lot about themselves. They wore the colours of their coats of arms, which announced their family history. But it didn't immiediately show whose side you were on! It would be like playing in a football match (a pretty lethal one) where every player is wearing a different strip. In those days of chivalry, knights at least were expected to recognize the colours and emblems of other knights. They clearly felt it was more important to tell people "I'm the Duke of Cornwall" or "I'm the Duke of Anjou" than to say "I'm fighting for the English" or the French.

The ancient Britons had a much simpler uniform when they fought the Roman invaders around the time of Julius Caesar: they painted it straight onto their bodies. [pic] It was a blue dye called woad, and it made them look fearsome. Julius Caesar said: "All Britons dye themselves with woad which makes them blue, in order that in battle their appearance is more terrible."

Colour wasn't just important in military life, but also in civilian and religious life. [Pic of Thomas More & family, early 16th C, showing red- and black-robed figures]. Point these out, and ask: Why red? Why black? We'll see the answer shortly.

And look at this painting. It is by the Italian artist Titian, who lived in the sixteenth century and painted this over the course of almost ten years. It shows the Virgin Mary and various saints, as well as some of the members of the rich family, the Pesaros, who paid Titian to paint it. It's not that the Pesaros simply wanted a nice picture: wealthy people in those days often paid a painter to paint a religious scene to decorate a church or an altar, which would show how devoutly religious they were-it was like giving an offering to God. But you see that they were often vain enough to ask to be included in the picture. The main point I want to make here, however, is that the colours in this painting aren't just chosen according to what took Titian's fancy. They have a symbolic significance. To the priests and educated nobles who would look at this picture, the colours carry particular meanings, although these are usually lost on us when we view paintings like this today. For example, the fact that this old chap here is wearing blue and yellow robes tells us that he is St Peter: painters in Titian's time had colour codes for important people like the saints, so that viewers could tell who they were.

Now, what I want to explain to you tonight is that a lot of this colour symbolism comes from a question of chemistry. What the colours mean often depends on the chemical substances used to make them.

First, let's have a look at what colour is.

Scientists and philosophers and artists have argued about this for centuries. The ancient Greeks, for instance, used to believe that colour was an intrinsic property of an object. What I mean by that is, just as an object has a certain weight, size, texture, smell and so on, it was thought to have a definite colour. In many ways we still think like this. We say that we've got a blue coat, or a green bike, or a red car. But what colour is the coat when it's in the cupboard? What colour is the car at night? You might say, well, it's still red. But under a streetlight, it might not look red at all - more a kind of orange-brown, maybe. What colour is it in moonlight? You've probably all seen how difficult it is to tell the colour of some cars at night. But of course, you could say that their colour is precisely what you see, not what you think you ought to see if it was the middle of the day and not night time. So you see, colour is not just about what an object is made of; it is about how it is lit too. Colour is about light.

Even in ancient times, people knew that colour can be conjured up from light. When a sunbeam, which is generally invisible and seems to have no colour itself, passes through a thick piece of glass or a glass of water, it can produce colour [pic]. This is called a spectrum.

The spectrum contains the same colours as a rainbow, and in the same order. The Wizard of Oz acknowledges this. When Dorothy is whisked to 'somewhere over the rainbow', she goes from a black and white Kansas to a Technicolor Land of Oz.

The first person to explain where the rainbow's colours come from was Sir Isaac Newton, in about 1665. He did a clever experiment to figure out what the colours of the rainbow are and where they come from. Newton let a single shaft of sunlight come through a crack into a darkened room, and pass through a prism. A prism is a piece of transparent material shaped like a Toblerone - now we usually make them out of glass, but in Newton's day they were sometimes made from clear crystals like quartz. Now, people already knew that if you do this, you get a multicoloured spectrum. They thought that the prism must be doing something to the sunlight, somehow changing it to give it colour - like sending a car body through the paint sprayer on an assembly line so that it comes out coloured.

But Newton did something more. He passed the coloured light through a lens, which focused all the rays back together again. And he found that where they came together, there was a spot of white light, just like the one you get if you simply let the ray of sunlight fall onto a piece of paper. In other words, the beam of light coming through the crack already contains all the rainbow's colours, and all the prism does is separate them out. If you mix them all together again, you get back white light.

Then Newton did another experiment. He put a mask-a piece of card, say-in the way of the spectrum so that only one colour of light could pass: say, red. He then passed this light through another prism. If colour really does come from something that prisms do to light, you'd expect this red light to be altered somehow when it passes through the second prism. But it isn't: it emerges again as red light. You can't break down this red light into any more colours. The same with all the other bands in the spectrum. So these are the fundamental colours - they aren't themselves composed of any other colours.

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