What is Delta E in colour matching?

Colour disputes are usually disputes about language. One person says a reprint does not match; the other says it is within normal variation. Both are describing the same sheets and neither has a way to settle it. ΔE replaces the adjectives with a measurement taken by instrument.

As a rough guide to the scale: a ΔE of around 1 is at the threshold of what a trained eye can detect side by side. Around 2 is noticeable on close comparison. Beyond about 3 to 5 the difference is apparent to most people without a reference to compare against. Exactly where an acceptable line sits depends on the job — a corporate identity colour across a stationery set is judged far more tightly than a background tint on a leaflet.

To use it, both parties agree a tolerance in advance, and printed sheets are measured against the approved reference with a spectrophotometer during the run rather than judged by eye under whatever light happens to be available. That is the meaningful part: instrument measurement removes the lighting conditions, the time of day and the observer from the argument.

It is most worth specifying on repeat work, on ranges where several products have to match each other, and on regulated packaging where consistency is part of the quality record.