Gray scale.

The reference scale that turns a colorfastness result into a grade: chip pairs set at defined contrasts, held against the difference between a tested and an untested specimen. Two of them exist, they answer different questions, and they are not built the same: the color change scale (AATCC EP1, ISO 105-A02) pairs two greys, while the staining scale (AATCC EP2, ISO 105-A03) pairs a gray chip with a white one. The essential ISO scale runs five steps, an augmented version adds four half-steps for nine in total, and AATCC spells it gray where ISO spells it gray. Chips fade with light and handling, so a tired scale quietly shifts the pass line your customer's spec is written against, and a lot you graded as passing comes back as a rejected shipment.