Colour measurement means using instruments that measure colour exactly, so that a printer, a paint maker or a textile mill can match the same shade batch after batch. The human eye is a poor judge. The same cloth looks different under a tube light and in sunlight, and two people may disagree about whether two reds match.
The instrument is called a spectrophotometer. It shines light on a sample and measures how much light the sample reflects at each wavelength. From the readings, the software calculates three numbers that describe the colour: how light or dark it is, how red or green, and how yellow or blue. The software then compares these numbers with those of the approved sample, which is called the standard. The gap between the two is a single number, usually called delta E. A delta E below about 1 is a difference that most people cannot see.
Consider a textile mill in Surat that dyes 5,000 metres of cloth in one batch. The buyer's approved shade is a certain blue. The operator holds the instrument against the dyed cloth, and the screen shows a delta E of 0.6. The batch passes. The next batch shows 2.3, a difference the buyer would see, so the mill re-dyes it before it ships.
The mill also uses the instrument earlier. Before dyeing the whole batch, it dyes a small cloth sample, called a lab dip, and measures it. If the number is off, the dye recipe is changed on paper, at the cost of a few grams of dye, not 5,000 metres of cloth. The instrument also handles a problem called metamerism, in which two samples match under one light but not under another. It measures under several lights, such as daylight and shop lighting, and so the shade is checked for the way the buyer will actually see it.
The same idea is used by printers, which must keep a brand's red the same on every pack. For that reason colour instruments connect to printers. They are also used with speciality chemicals, such as dyes and coatings, whose makers need to check each batch. The scene around them is materials science, and colour is one of the properties that advanced materials must have exactly right.