The judgement everybody makes first
Looking into the pot and deciding whether the tea is ready is nearly universal, and it is entirely understandable — colour is the only variable available without tasting, it changes visibly, and it changes on roughly the right timescale. It is also the reason a great many people brew a pale tea for far too long and a dark one for not nearly long enough. The habit is reinforced by the language available: a good cup is described as dark, a poor one as pale, and strong and dark are used as though they were synonyms in ordinary speech. They are not, and the teas where they come apart are not rare exceptions to be noted in a footnote but entire categories that people drink every day. Anyone who has poured a properly made silver-needle white tea and been told it looks like hot water has met the problem, and anyone who has been served a jet-black cup of shou pu-erh with no body to it has met the other half of it.
What colour actually tracks
The colour of tea liquor comes largely from the oxidised polyphenol compounds formed during processing, together with whatever the water itself contributes through its mineral content and alkalinity. That means depth of colour is chiefly a measure of how much of one particular fraction has been extracted — a fraction that is abundant in black and dark teas and scarce in green and white ones — rather than a measure of dissolved material in general. It correlates only loosely with body, hardly at all with sweetness or aroma, and unreliably with caffeine content. Two liquors of identical depth can differ enormously in everything a drinker cares about, and two liquors of very different depth can be equally intense in the mouth. That is exactly what makes the visual judgement unreliable rather than merely imprecise: it is not a noisy measurement of the right thing, it is a clean measurement of the wrong thing.
Four cases where the correlation breaks
Silver-needle white tea brewed properly is barely tinted and can be thick, sweet and long-finishing, so anyone waiting for colour will steep it into astringency. Shou pu-erh goes almost black within seconds and may still be thin, because the colour comes out far faster than the body does and the two are not on the same schedule. Green tea brewed in alkaline water darkens noticeably while tasting duller rather than stronger, which means the water has changed the colour without changing the extraction in the same direction — a particularly confusing case, because the drinker did nothing differently. And cold-brewed tea of any category is much paler than its hot equivalent while carrying a full load of sweetness and aroma, which is why people cold-brewing for the first time routinely conclude it has failed and add more leaf. In every one of these, someone brewing by eye reaches the wrong conclusion confidently.
Why colour arrives before the tea does
In a boiled or long-steeped dark tea the sequence is particularly misleading. The liquor reaches something close to its final depth within a minute or two, and the body — the thickness that is the entire reason for boiling it — carries on developing for several minutes afterwards. Anyone stopping when it looks right stops several minutes early and gets a coloured liquid with no weight to it, which is then remembered as what boiled dark tea is like. The same ordering applies at smaller scale in an ordinary steep and in the early infusions of a gongfu session, where the second infusion often looks lighter than the first while tasting bigger. This is why the parameter sets in this catalogue for boiled and compressed teas say explicitly to judge by body rather than by colour, and say that the colour arrives first: it is the one place where a written instruction can head off a misreading that the eye will otherwise make every time.
What to judge instead
Taste it. That is the whole answer, it is available in every method that pours into a cup, and it is faster than any proxy. Failing that — in a method where the leaf cannot be sampled mid-steep, or when brewing for other people — the useful indicators are weight in the mouth rather than depth in the pot, the length of the finish after swallowing, and the smell of the wet leaf, which costs nothing to check and tells you a great deal about where a session has got to. In a multi-infusion session the most reliable single indicator is the aroma of the drained leaf between infusions: it reports on what is left rather than on what has come out, which is precisely the question being asked. Experienced drinkers use it constantly while appearing not to look at the liquor at all, and it is the habit most worth copying from watching someone who knows what they are doing.
Colour still tells you something
It is not useless; it is answering a different question from the one it is usually asked. Consistency of colour across a session is a genuine signal about how evenly the leaf is extracting, and a sudden lightening after an infusion that was poured incompletely tells you the vessel was not drained. Cloudiness in a cooling black tea is the haze that forms as certain compounds associate on slow cooling, which is cosmetic rather than a fault and is fixed by chilling faster rather than by brewing differently. A green tea that comes out yellow-brown rather than green has usually met water that was too hot or has been standing too long, and there colour is genuinely diagnostic. And an unexpected colour from a familiar tea is worth investigating, since it often means the water has changed. The rule worth carrying is that colour reports on process and almost never on strength.