Green: Why did nature's color become such a difficult and dangerous dye in history?

Short answer

Green is the most abundant color in our surroundings, but carrying the green of plants onto a fabric or a wall was very hard: the chlorophyll that gives a leaf its color doesn't make a lasting dye, and there was almost no natural dye that gave a lasting green on its own. Dyers obtained green by overdyeing blue with yellow; a bright, cheap green, by contrast, arrived with arsenic at the end of the eighteenth century and poisoned people. Today, in reverse of this complicated past, the same color is the color of safety and exit in standards.

Science — why is green everywhere?

A leaf is green because the chlorophyll molecules that collect light in photosynthesis strongly absorb the blue and red regions of the spectrum and send back more of the green in between. Why a leaf doesn't absorb all the light and turn black — in other words, why green is the "rejected" color — is still a debated and interesting question, and it is the subject of DO-04.

Green has another property: our eyes are very sensitive to it. In daytime vision, among lights of equal energy, the human eye perceives a yellowish green at around 555 nanometers as the brightest. That's why green light becomes visible even at low energy. For a blue or red lamp to look equally bright, it has to emit more light than a green lamp; this difference is taken into account in lighting design and display engineering.

But separating a leaf's green from it and preserving it is almost impossible. Chlorophyll breaks down quickly in light and air; a dried leaf turns yellow, then brown. Green, which is everywhere in nature, is one of the hardest colors to extract from nature. Why leaves turn yellow in autumn is tied to this same fragility, and it is explained in DO-05.

In the animal kingdom, too, green is often not a single pigment. The green of many parrots and frogs comes from a yellow pigment layered over a structural blue produced by the fine structure of the skin or feathers: the yellow filter turns the structural blue into green. Nature, too, produces green by combining two colors, just like medieval dyers. How color without pigment is produced is explained in MA-08.

History — how was green obtained?

The most common way to dye fabric green was to use two dyes in succession. The fabric was first dipped in a blue such as indigo or woad, then in a yellow such as weld. The green fabric that dyers in the English city of Lincoln produced with this method became so famous that the color's name came to be linked with the city. The method was laborious and expensive; and when the two dyes faded at different rates, the fabric would drift toward blue or yellow over time.

Painters didn't have it easy either. Greens made from copper compounds were bright, but some would darken over time and react with other pigments. A bright, cheap, and durable green was therefore a major gap.

The Swedish chemist Carl Wilhelm Scheele filled that gap in 1775, but at a heavy cost. The green pigment he made from copper and arsenic quickly found its way into wallpaper, clothing, artificial flowers, and even candy. A brighter version developed in 1814 in the German city of Schweinfurt spread under the names Paris green and emerald green. In damp rooms, arsenic-laden dust and gases released from wallpaper made people ill. How widely these greens spread, and why they were banned so late, is the subject of TK-07.

Culture — what did green mean?

Green's meaning was as unstable as its substance. The French medievalist Michel Pastoureau argues that in Europe, green followed this path: because its dye was unstable, green became associated with everything changeable and fleeting — childhood, love, hope, luck, games, and money. Pastoureau's account is a historian's interpretation focused on Europe; in other cultures, green's story is told differently.

Turkish also connects green to freshness. The word yeşil ("green") comes from yaşıl, derived from Old Turkic yaş, meaning tender, fresh, and moist; a fresh branch or a damp cloth that Turkish still calls yaş ("fresh/moist") carries the origin of the color's name. The same association appears in two different traditions: green is the color of what is not yet ripe, of what is still changing.

In Islamic art and tradition, by contrast, green holds a very different, honored place; it's associated with depictions of paradise and with the decoration of sacred spaces. The lesson that compares the meanings of colors across faith traditions in neutral language is TK-15. At the end of the twentieth century, green became the color of environmental movements and of political parties bearing that name; how a color becomes attached to a political movement is addressed in TK-17.

Today — where is green, and for how much?

Today green is a color bound by rules. It shows "go" in traffic, and in workplace safety signs it shows a safe condition: the emergency exit, the first-aid cabinet, the assembly point. It's no accident that you see a green sign with a running human figure in a dark corridor; international standards set this visual language, and the details are the work of EN-07.

Green light's relationship with skin tone, however, is often an unwelcome side. Older fluorescent lamps can cast a green shift, and under that light every skin tone looks pale and sickly. Photographers and lighting crews correct this shift with a filter in the opposite color; green–magenta correction is the subject of FS-12.

Pigments, as far as green goes, have left their dangerous days behind. Arsenical greens were replaced by durable inorganic greens such as chromium oxide and, in the 20th century, by strong organic pigments such as phthalocyanine green; the chemistry of these families is covered in MA-03 and MA-04. Nature's color could only become both cheap and safe in the laboratory.

In the market, green today does different work: it's used on packaging as a promise of naturalness, being organic, and environmental friendliness. How this code settles in category by category is discussed in PZ-04; it's enough to remember that a product's green packaging is not, by itself, evidence about its contents. Green's story is the story of a color that carries its meaning independent of its substance: the color that once carried poison now promises safety and nature. The next time you look at an emergency exit sign, you can see three separate stories layered on top of each other: the eye's high sensitivity to green, the one-time transformation of a poisonous pigment into a safe one, and a standard that officially defines this color as "safe."

Common misconception

Summary

1. Plants are green because chlorophyll absorbs blue and red; but because chlorophyll isn't a lasting dye, nature's green can't easily be extracted.
2. Dyers obtained green by overdyeing blue with yellow; a bright, cheap green, by contrast, arrived with arsenic starting in 1775 and poisoned people.
3. As an unstable color, green was associated with change in Europe and with sanctity in other traditions; today it's the color of safe conditions and exits.

Test yourself

1. (Easy) Which pigment makes plants appear green?
2. (Easy) Which element makes Scheele's green toxic?
3. (Medium) Where will you first notice this color in your surroundings today, and what will you now see differently?
4. (Medium) How did medieval dyers obtain a lasting green fabric?
5. (Hard) What connection does Pastoureau draw between green's chemical instability and its cultural meanings, and what limit should be kept in mind when reading this interpretation?
6. (Hard) What does the eye's peak sensitivity to green suggest, and not suggest, about why emergency exit signs are chosen to be green?

Answers

1. Chlorophyll; it absorbs blue and red light and reflects more green ("Science").
2. Arsenic; the pigment is a compound of copper and arsenic ("History").
3. Everyone's answer will be different: the exit sign in a corridor, a plant by a window, or fluorescent light in an office. When evaluating, look at this: is that green the product of a rule (a standard), a substance (a pigment, chlorophyll), or a light — and which of these three layers had you never thought about before ("Today").
4. By dyeing the fabric first blue with indigo or woad, then yellow with something like weld — that is, by overdyeing ("History").
5. He says that because green pigments were unstable for a long time, green became associated with the changeable and the fleeting (luck, love, money). This interpretation is focused on Europe; in other traditions, green's meaning is different ("Culture").
6. Green looking good even in low light may be a property that supports the choice; but the color of signage is also set by international standards and habit, and can't be explained by eye sensitivity alone ("Science", "Today").

Sources

• Pastoureau, M., Green: The History of a Color, Princeton University Press, 2014.
• Maryland Center for History and Culture, "An Update on Arsenic Green: When the World was Dying for Color."
• U.S. National Library of Medicine, Circulating Now, "Facts and Inferences — Digitizing Shadows from the Walls of Death."
• RP Photonics Encyclopedia, "Luminosity Functions."
• Storied Colors, "Lincoln Green."
• Nişanyan Sözlük, entry yeşil.

Next step

DO-04 deepens this lesson's science act: it covers the absorption spectrum of chlorophyll a and b, how light is used in photosynthesis, and the hypotheses for why plants don't absorb all light and turn black. TK-07, meanwhile, opens up the dark side of the history act: it follows the spread of Scheele's green into wallpaper and clothing, the contested theory about Napoleon's death, and how regulations came to be.