Foundation ranges attract sharper criticism than almost any other beauty product. The reason is that shade matching involves two independent variables and most ranges only handle one well.
Depth and undertone are separate axes
Depth describes how light or dark a skin tone is, and it is the axis most obviously visible when shades are lined up on a shelf.
Undertone describes the underlying colour cast, whether cooler and pinker, warmer and more golden, or more neutral and olive.
Two people at identical depth can require entirely different shades, which is why counting the number of bottles says little about coverage of the range.
Ranges tend to collapse at the extremes
Manufacturers often add depth by increasing the proportion of darker pigment in an existing base, which shifts depth without adjusting undertone properly.
The result is that the deepest shades in a range frequently share one undertone, usually red or orange, leaving cooler and neutral deep tones unserved.
The same compression happens at the lightest end, where very fair shades often skew pink and leave fair olive tones without a match.
Olive undertones fall between categories
Olive skin carries a green or grey cast that neither the warm nor cool designation describes, and it appears across a wide range of depths.
Foundations built on a simple warm-to-cool scale therefore look either too pink or too orange on olive skin regardless of which depth is chosen.
Correcting this requires adding a distinct pigment rather than adjusting the ratio of existing ones, which is a formulation change rather than an extension.
Oxidation moves the target
A shade that matches on application can shift warmer over several hours as pigments react with air and skin oil, which is a separate problem from initial matching.
Because the shift is towards orange, it affects those already served by warm-leaning ranges least and those needing neutral or cool shades most.
A range can therefore appear well matched in a store and still fail in ordinary wear for a substantial part of its intended customers.
Why the criticism is precise rather than general
Reviewers assessing a range typically test whether the deepest and lightest shades vary in undertone as well as depth, which is the specific failure being identified.
A range with fewer shades that vary across both axes serves more people than a longer one that varies mostly in depth.
Judging by count alone therefore misses the point of the criticism, which concerns the structure of the range rather than its length.