Every mass-produced colour exists as a family of variants rather than a single value. Factories run several paint lines, tinters drift between batches, and a metallic sprayed in Zuffenhausen in March is not identical to the same code sprayed there in October. Then the car spends six years in daylight. By the time it reaches us, a silver may sit two or three perceptible steps away from the code it was born with — and a body shop that mixes straight to the code and sprays a single panel has just made that difference permanent and put it in your eyeline for the rest of the car’s life.

A spectrophotometer takes several readings from a clean part of the panel next to the repair, at different angles, because metallics and pearls change with viewing geometry. The instrument returns a formula from the variant library; Ian adjusts it by hand from raw tinters, because the instrument is very good and not quite finished. That mix is sprayed onto a test card with the same gun, the same pressure and the same number of coats as the car will get, baked, and then held against the panel in daylight rather than under the booth lights. If the difference is above 0.4 ΔE it goes back on the scales. Most mixes take two attempts; some silvers take five.

Even a perfect mix is sprayed by a person onto a curved surface, so on anything metallic we blend into the adjacent panels rather than stopping hard at the shut line. A blend puts the transition in the middle of a panel where the eye has no edge to compare against, instead of at a gap where it has a perfect one. It costs more in materials and time and it is the difference between a repair you cannot find and one you can see from the kerb on a bright day. It is included in our panel rate; if a quote you are comparing is markedly cheaper, blending is the first thing to ask about.

A solid white or black is the easiest match and the hardest finish, because there is nothing in it to hide a flaw. Three-stage pearls — a ground coat, a translucent pearl, then clear — are the reverse: they are forgiving of texture and merciless about coverage, since the depth of pearl is a function of how many coats went on, so the test card has to replicate the exact number. Some period colours from the seventies have no digital record at all; those get matched from a cut panel under a daylight lamp, by eye, which is a skill rather than a process and takes as long as it takes.

Look along a panel rather than at it, from a low angle with the light behind you — reflections tell you far more than colour does. A good blend has no edge anywhere; if you can find a line where one panel stops and the next begins on a metallic, it was sprayed panel-to-edge rather than blended. Open the door and look at the shut faces and the jamb: a shop that painted the visible face and left the jamb its original shade has told you exactly how much of the job it did. Run a fingertip over the lacquer for nibs of trapped dust. And ask to see the film-thickness readings — any workshop that gauges paint will have them, and any workshop that does not gauge paint has been guessing at what lies underneath.