Colour Metrology
CMC tolerance is an ellipsoid, not a sphere. The same numerical tolerance is a much tighter box on a dark dull standard than on a bright one.
— ΔL*
Half-width of the acceptance ellipsoid in lightness
Valid for L* at or above 16; below that SL takes a different form and this will understate the lightness axis. The l:c ratio encodes what the eye tolerates in a given end use — 2:1 is the usual textile setting — while the commercial factor is the business decision layered on top. Agree both with the customer in writing before a lot is judged.
CMC(l:c) Tolerance Ellipsoid Boundary Calculator — free, with the formula and a worked example, at Textile School.