So Why is Green Curry Green?

Green curry is green because fresh green chillies are pounded into the paste. The pigment is chlorophyll, and it sits in the chilli's skin and flesh, so the colour of the finished curry is a read-out of two things only: how many fresh green chillies went in, and how hard the paste was cooked afterwards. Everything else in the paste is pale. Lemongrass, galangal, garlic, shallot and coriander root are cream or white, the ground cumin and coriander are brown, and the shrimp paste is grey.

There is a second half nobody tells you before you cook it. A green curry made the way Thai kitchens make it is not bright green. It is olive, khaki, a little dull, with green-gold oil floating on top in small islands. That is the correct colour, and it comes from the same chemistry: chlorophyll gives the paste its green in the mortar, and the pan takes most of it away.

What makes green curry green?

Start with the arithmetic. Every printed green curry paste we can check puts chillies in by handful and everything else in by teaspoon.

The paste in Food of Thailand is "8-10 small green chillies, seeded" against a 2.5 cm piece of galangal, a teaspoon of chopped makrut lime skin, 3-4 shallots, "5-6 coriander (cilantro) roots", a handful of holy basil, and 2 teaspoons of shrimp paste. The teaching recipe in WCA Thailand Recipes, the Culinary Institute of America book written with Marnee Thai in San Francisco, is the clearest professional version: four dark green jalapeño or serrano peppers and 15-20 fresh green Thai chillies against a teaspoon of kaffir lime zest, two teaspoons of coriander root and two of shrimp paste. Chillies by weight, aromatics by teaspoon.

So the pigment in the paste is chilli pigment. Pounding does not create the colour, it releases it: the pestle breaks the cells, the pigment spreads through the oil, and a pile of separate ingredients becomes one mass. That is the moment the colour is strongest, and everything after it is a loss. My method is in my guide to how to make Thai green curry paste.

Chlorophyll holds its green because a magnesium atom sits at the centre of its ring. Heat and acid push the magnesium out and hydrogen takes its place, and the molecule becomes pheophytin, which is olive-brown. That one reaction explains most of what happens between the mortar and the table. It is the same one that turns boiled greens from bright to army green, it is measured in cooking-science studies as the reason heat-processed green vegetables fade, and the chlorophyll explainer many of you have already reached from a search covers the biology.

Two things follow, and that is where home cooking goes wrong. The acids that drive the reaction are not only the ones you add: they are inside the chillies and herbs already, and cooking frees them. And colour and heat are separate dials, because the recipe above gets the bulk of its colour from four mild green chillies and its heat from a separate pile of bird's eyes. One of the books on our shelf calls its green curry with chicken "this familiar classic, which should never be extremely hot", and Sebby Holmes of Farang in London is exact about the comparison everyone argues over: swapping dried red for fresh green "shouldn't make much difference at all to the spice levels; in fact, the use of bird's eyes makes for a spicier curry". Our page on red versus green Thai curry works the heat through.

Is green curry green from coriander and kaffir lime leaves?

Less than the recipes say. National Geographic Traveller took it apart in its green curry feature: "a quick internet search suggests up to half of English-language recipes include them, especially those by non-Thai celebrity chefs", meaning coriander leaves. In Thailand they do not go into the paste at all: "coriander root or, at a push, a few finely chopped stalks should feature in the paste. The leaves don't just taste wrong, they also float in the broth and oxidise to an unpleasant brown within hours of being ground to a paste." That last sentence is the colour answer. A ground coriander leaf browns within hours, which is a polite way of saying that coriander leaf in your paste makes the curry browner, not greener, the longer it sits.

The printed recipes in our corpus follow the Thai practice without exception: "5-6 coriander (cilantro) roots" in Food of Thailand, "1 tsp chopped coriander root" in Thai Cottage, and "Cilantro roots, finely chopped 2 tsp. or substitute with 1 Tbsp. chopped stems" in the Culinary Institute recipe. Labels that name the herb get the part right too: Thai Kitchen's green curry paste lists "Coriander Root & Kaffir Lime Peel". The jars that carry the leaf are doing something else, and their lists say so, since Blue Dragon's UK jar ends with coriander leaf and "Colour (Chlorophylls)". National Geographic reaches the same conclusion from the supermarket aisle: coriander leaves appear "as a main ingredient in many curry pastes in British supermarkets, along with added chlorophyll for a brighter hue".

Kaffir lime is a smaller version of the same mistake: the zest is green and belongs in the paste, but a teaspoon of it tints the edges rather than the body of the colour, and its work is perfume. Our page on kaffir lime leaves covers zest and leaf.

Green, red or yellow: where each curry paste gets its colour

In Thai the dish is gaeng khiao wan, แกงเขียวหวาน, which does translate as "sweet green curry". That is where the confusion about its flavour starts: in Thai, "sweet green" names the pale colour, not the taste. The chilli in it is prik khi nu, mouse-dropping chilli, which is the bird's eye.

The three pastes an English-speaking cook meets are one recipe with one part changed: the chilli. "Red and green curry pastes are pretty much exactly the same, but the key difference is that dried red chillies are used in a red, as opposed to fresh green bird's eye chillies in a green curry," Sebby Holmes told National Geographic. Green curry is the newcomer, not a separate bloodline: the historian Hanuman Aspler traces its first appearance in print to "a 1926 book by an author with the pen name L Phaehtraarat".

Red paste is built on dried red chillies. The Complete Asian Cookbook's Bangkok red curry paste starts with "4-6 dried red chillies" and then adds two things that are there for colour rather than heat: "1 teaspoon ground turmeric" and "2 teaspoons paprika". Its glossary is honest about the cheat: paprika is "useful in Asian cooking for imparting the necessary red colour to a curry when the chilli tolerance of the diners is not very high. In Asia, the colour would come from 20 to 30 chillies!"

Yellow paste is not made from a yellow chilli, because this cuisine has no such thing. Thai Cottage's "Yellow curry paste (Nam-phrik Gaeng Ka-ri)" is built on "7 chilies chopped red dried chilies" with a teaspoon of chopped turmeric, the Thai khamin, and half a teaspoon of curry powder. The same book states the mechanism plainly: "Turmeric provides a bright yellow colour to some Thai curries." Green paste is the only one whose colour comes from a live ingredient, and the Thai Cottage green curry paste shows how loose that boundary is: it carries "1 tsp chopped turmeric" too, which is one way a home curry comes out yellow-green rather than green.

What matters is not the hue but how each pigment behaves under heat. The red in dried chillies is carried by carotenoids, chiefly capsanthin and capsorubin, which occur only in chillies and give paprika its red. Turmeric's yellow is carried by curcuminoids. Both dissolve in fat and both are patient with heat, which is why a red or yellow curry gets deeper and glossier the longer you fry its paste. Chlorophyll is fat-soluble too, but it is not patient: heat and acid strip the magnesium out of the ring, and it goes olive, then brown. Green is the only one of the three colours that can be destroyed by cooking the dish properly, which is why "fresh" is a technical requirement here rather than a marketing word.

Why is my green curry not green?

In order of how often it is the cause.

1. The paste was fried too hard. Frying the paste in thick coconut cream is correct and necessary, but frying it until it darkens is where the colour goes, because garlic, shallot and shrimp paste carry sugars and amino acids, and once they catch they brown the whole sauce with them. Look for the oil splitting out and the raw smell of garlic and shallot cooking off, not for a darker paste. This page on a grainy, separated curry works that step through in full.

2. It simmered too long and too fast. The pheophytin reaction, given time. A green curry is not a braise: vegetables go in at the end and the whole thing is on the hob for minutes. A hard boil costs you the colour and the emulsion at once, and the same thing happens again in a pan kept hot on a buffet, which is why the second serving is always browner.

3. The chillies were not fresh, or were ripe. A green bird's eye chilli is the unripe fruit of the same pepper that ripens to red, and it loses chlorophyll as it ripens. A chilli gone soft, dull or blushed in the fridge door has less colour in it than the day you bought it. If they are going soft before you cook, they need to be in a paste first: see how to store fresh Thai chillies.

4. You used a jar and expected emerald. Blue Dragon's UK label runs: water, lemongrass 11%, garlic purée 11%, sugar, oil, galangal, modified starch, green bird's eye chillies 3%, Thai basil 2.5%, coriander leaf, lime leaves 1.5%, spices including turmeric, colour from chlorophylls. Three per cent chilli, a yellow spice in the list, and a leaf-derived colour put back at the end because processing has taken the green out. Andy Oliver, head chef at Som Saa, puts it from the other side: green curry paste is made of "fundamentally fresh ingredients", so "something sitting on a shelf with a two-year shelf life is a serious compromise". A jar gives a khaki curry whatever you do to it; that is the product, not your cooking. To get the colour back, make the paste, and keep it a short time in the fridge, because a homemade paste dulls as it ages for the same reason the curry does.

5. It was blended hard rather than pounded. A food processor runs hot and whips air into the paste, and heat and oxygen both attack chlorophyll before the paste meets a pan.

What does a properly cooked green curry look like?

Olive. Khaki. Slightly muddy in the body of the sauce, with small green-gold islands of oil on the surface.

That oil is the useful part, and it is not decoration. Because chlorophyll dissolves in fat rather than water, the pigment leaves the paste with the coconut fat as the cream cracks, and comes up through the sauce tinted green. National Geographic's feature makes it the test of the whole method: "green dots of oil should decorate the surface of the broth, as this shows the paste has released all its flavour". Read the oil, not the sauce. Green oil means the paste cooked properly in a sauce that has lost most of its bright pigment, which is exactly what should happen.

People who cook this dish are blunt about the honest colour. The Bangkok food writer behind Bangkok Glutton records a chef friend's name for the ideal restaurant colour, a "lovely celadon" he calls "sexy green", then says he does not trust such curries, because the one he is eating at an aunt's kitchen in Bangkok is what the late Andy Ricker would call "khaki". A home cook of Thai curries writes that "khaki green is actually the true color of a traditional green curry", and that a curry vivid enough to be called pistachio, mint or emerald has almost certainly had food colouring in it. The writer of a vegan Thai green curry puts the motive best: "Gaeng keow wan is a khaki coloured soup dappled with green oil. If you have ever been served a bright green curry, it's due to unnecessary blending of coriander, pepper leaves or (less offensively) Thai basil. I can only assume this is to meet customer's demands when they expect emerald upon reading the word green."

So a takeaway curry the colour of mint sauce is either a leaf-derived colour, which several UK jars declare on their labels, or leaf matter blended into the paste. Neither is a crime, and neither is what the dish looks like when a Thai kitchen cooks it. Do not confuse a dull colour with a broken sauce, though: faded is normal, while split and grainy is a technique fault, and the two are fixed at opposite ends of the pan.

How to keep a green curry green

1. Buy for colour. Firm, glossy, fully green chillies, and put any that have started to blush into a dish where colour does not matter.
2. Pound where you can. A mortar stays cool and barely aerates the paste. If you blend, use short pulses and stop while the paste is still cold.
3. Fry the paste in the thick cream only until it is fragrant and the oil splits, then stop. Three to five minutes, not fifteen.
4. Keep the simmer gentle and short, and put the vegetables in at the end.
5. Add the fresh green at the very end, off the heat: sliced bird's eye chillies and Thai basil. The Complete Asian Cookbook's green curry of chicken says it directly, and it is the line worth following literally: "The green colour is imparted by the finely chopped chillies and fresh herbs added during the last few minutes of cooking."
6. Serve it soon, and reheat once, gently, with a splash of fresh coconut milk.

Then judge it properly. The green curry I cook is written to land exactly here, khaki sauce and green oil and all. If the oil is green and the sauce is khaki, you have cooked it well, and the difference between that and a bright green curry is not skill but colouring.

A note on sources

The ingredient lists quoted here are printed recipes, quoted from four books we hold on file: Food of Thailand, Thai Recipes from Thai Cottage, The Complete Asian Cookbook and WCA Thailand Recipes. Two of them carry no page numbers in the text we have, so their recipes are named rather than paginated. The coriander-leaf claim, the green-oil test, the 1926 first-print date and the quotes from Sebby Holmes, Andy Oliver and Hanuman Aspler are from National Geographic Traveller's green curry feature. The khaki-colour material above comes from three writers who cook this dish: Bangkok Glutton, The Spice Odyssey and Messy Vegan Cook. The jar lists are Blue Dragon's and Thai Kitchen's UK product pages as printed at the time of writing, and labels change, so read the jar you bought. The pigment chemistry is standard food science. Nothing was cooked for this page, and the jar arithmetic is a label read, not a laboratory analysis.

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