Most readers of the Literary Review will know that milk comes from cows and that dyes once came from plants. But do they realise that scarlet and crimson came only from insects – from the crushed bodies of millions of bean-sized bugs? They were scale insects, relatives of the pests that devour their way through a conservatory cactus garden.

In the Middle Ages, dyers had access to only three of these dye-producing insects. There was kermes, which lived in a special oak around the shores of the Mediterranean; Polish cochineal, which lived in the roots of a plant that grew sparsely in the Baltic marshes; and Armenian cochineal, which came from the marshes at the foot of Mount Ararat. Armenian cochineal produced the most costly and desirable colour, but the insects appeared only once a year, in a blood-red carpet that stained the fleece of passing sheep.

The rarity and expense of the red dyes produced from these insects gave them a special cachet in medieval society. But there were other factors too, both cultural and physiological. Red is the primal colour. The human eye is highly sensitive to red (hence red traffic lights), and in most languages the word for red is older than any other colour term, except black and white. Often it is also synonymous with ‘beautiful’. Red is the colour of fire and the sun, givers of life, but also of blood, and so of death, courage, and martyrdom.

As a symbol of authority, red inherited the mantle of imperial purple, that unbelievably expensive dye that the Romans made from shellfish. When the Ottomans captured Constantinople, they destroyed the last remaining purple dyeworks. So the Church, whose cardinals previously wore purple, adopted crimson instead.

Colour was then a prerogative of privilege. The poor wore drab greys and browns while the rich flaunted a brilliant plumage of red, purple, yellow, and blue. A fast, bright red had the kind of class that the perfect Savile Row suit has today, and as such it was fiercely protected by the dyers’ guilds.

So imagine the alarm of the dyers and the glint in the eyes of the merchants when it was rumoured in the 1520s that the Spanish had found a new red dye in Mexico. It was true. They had stumbled across another cochineal, a dye that was thirty times more powerful than Armenian cochineal and came from a local scale insect that the Mexicans had specially bred for size and colour.

The dyers did tests and concluded that this new dye was a good thing, while the Spanish crown established  a lucrative monopoly in cochineal. It became a major export from the New World, second only to gold and silver, and was traded all over Europe.

To the other European powers, it was intolerable that the Spanish should control this vital commodity. National prestige was at stake. How could a monarch conduct himself with pride in a red that came from Spanish territory? But power requires knowledge, and at this point hardly anybody in Europe knew what the cochineal was. Some thought it was a worm, others that it was a berry, or even a ‘wormberry’.

The Spanish guarded their secret carefully, and it was only in the 1720s that it was uncovered, through the combined efforts of the famous microscopist Antoni van Leeuwenhoek and an eccentric fellow-Dutchman, who made it the subject of a huge bet. Their conclusion was that the cochineal was indeed an insect and that it lived on the prickly pear, or nopal cactus.

The holy grail of eighteenth-century science was plant transfer, and some very great minds now applied themselves to the problem of rearing this insect outside Mexico. It was an almost impossible challenge, for the cochineal would live only on the nopal plant, and the plant quickly died in the wrong conditions.

The celebrated botanist Carl Linnaeus firmly believed that the cactus could be trained to adapt to the frozen north. A French expert, Nicolas-Joseph Thiery de Menonville, was more realistic and decided that the French colony of Saint-Domingue might be the solution. After a series of colourful adventures worthy of Patrick O’Brian’s Stephen Maturin, he succeeded in smuggling nopals and insects out of Mexico, but shortly afterwards he died. All that survived of his efforts was a treatise on the rearing of cochineal, dedicated to the ‘Monarch most worthy of wearing it … the greatest King on earth’.

In Britain, the country whose ‘redcoats’ were fast establishing the greatest empire on earth, the need for cochineal was particularly pressing. The men wore coats dyed with ordinary madder, but the officers had to have cochineal. (This made them an easy target for discerning sharpshooters, but it would be many decades before the British officer would relinquish his splendid scarlet jacket for drab khaki.) So Sir Joseph Banks, hoping to make the British Empire self-sufficient in all the major commodities, included cochineal in his elaborate programme of plant exchange.

Eventually, as the Spanish grip weakened, cochineal expanded − into Guatemala, Spain, the Canaries, Java and North Africa − and by the 1850s production was worth £2 million a year. But this great success came too late. In 1856 William Perkin discovered mauve, the first synthetic dye, and colour changed for ever – it became cheap. A century earlier, Jane Austen’s mother, the irreproachable wife of a country parson, had bustled around in a scarlet riding habit, but now scarlet especially was seen as sinful and tawdry. Black had become the garb of authority and respectability.

Today colour is no longer a precious commodity, hard won from nature, but a powder in a tub. Yet it still has a place in our culture. We register the symbolism of colour, even though we may not understand its origins. To find out why the Chancellor wears a bright red tie and teenage mums in Little Britain wear candy-pink tracksuits, read A Perfect Red. It is not quite a perfect book, combining as it does history ‘lite’ with lengthy digressions into the Central American encomienda system, but it is still an eye-opener.