Depth and undertone, asked separately, answered by you, inferred from nothing. Here is what that rules out and why.
The Fitzpatrick scale was built to set a phototherapy dose, not to describe skin.
It was developed to choose a starting ultraviolet dose for PUVA treatment, and the original paper describes the problem it solved as classifying persons with white skin whose tolerance could not be judged from appearance alone. It has since become the default way products and forms ask what your skin is, which is a job it was never designed for and has never been validated for.
Supports · That the scale's purpose was ultraviolet dosing, and that its original framing concerned white skin and sun reaction.
Does not establish · That the scale is useless for the dosing it was built for. This is about the job it has been given since, not about the job it does.
Archives of Dermatology →It does not encode undertone, and it is a poor proxy for measured skin colour.
Published criticism in the dermatology literature — including from dermatologists whose work is skin of colour — finds that Fitzpatrick type correlates poorly with objectively measured constitutive skin colour, that it is routinely used beyond its purpose as a stand-in for colour and for race, and that it does not capture baseline pigmentation, undertone or inflammatory response. That last one is the whole reason this desk asks two questions instead of one: depth and undertone are separate axes and a single phototype flattens them into each other.
Supports · That the criticism is documented in peer-reviewed and clinical literature, and that undertone specifically is named as something the scale does not carry.
Does not establish · A formal position statement from any professional body. No regulator or association has been found saying this in those words; the criticism is in the journals, made by clinicians, and that is the honest way to describe it.
Cutis →A tan-or-burn question fails hardest on the people it describes least.
A review of skin-colour assessment reports that between 40 and 60 percent of people could not select a Fitzpatrick category at all, and gives the reason plainly: the vocabulary does not describe how deeply pigmented skin behaves. Black participants in one cited study described their skin as getting darker rather than as tanning. A question that offers no answer you recognise is not a measurement, and this is why no form on this desk asks how you burn.
Supports · That self-classification breaks down at a large rate, and that the failure is a vocabulary failure specific to how the scale describes response.
Does not establish · That any particular alternative consumer test is validated. The review's own answer is instrumented colour measurement in a clinic, which is not a thing a reader can do at a bathroom mirror. The forty-to-sixty figure is quoted by that review from earlier work rather than measured in it.
Journal of the American Academy of Dermatology →How you react to sun and how pigmented you are are two different measurements.
When Fitzpatrick types were compared against instrumented pigmentation readings in 115 people, the correlation was poor, and people who self-reported as the fairest type spanned a pigmentation range from very light to intermediate. Sun reactiveness and constitutive pigmentation are measuring different things, which is why a burn-history question cannot deliver a colour fact.
Supports · That the two do not track each other, with a measured comparison behind it.
Does not establish · Anything about undertone. The instrument used here reads depth, not temperature, so this establishes that the tan question is the wrong question — not what the right one is.
Photochemistry and Photobiology →The objective measure the industry reaches for cannot express undertone either — by construction.
Individual Typology Angle is computed as the arctangent of (L* minus 50) over b*, converted to degrees. Only lightness and yellowness enter it. The a* axis, which is the one that carries red against green, is not in the formula at all — so a number that captures how light skin is and how yellow it is has nothing left to say about whether it is red-leaning or olive. That is not a criticism of the measure; it is a description of what it was built to measure.
Supports · The formula, from two independent sources, and that a* is excluded from it.
Does not establish · Why a* was left out — neither source explains the original rationale — nor that a* would be a sufficient encoding of undertone if it were included. The conclusion that ITA cannot express olive or red-leaning character follows from the formula rather than from a sentence anybody has published.
Bioengineering →The wrist-vein test is reading vessel depth, not your skin.
Why a vein looks blue was worked out properly in the optics literature and the answer is about the vessel rather than the skin: it depends on how the skin scatters and absorbs at different wavelengths, how oxygenated the blood is, and — decisively — how deep the vessel sits and how wide it is. A vein needs a minimum depth and a minimum diameter before it reads blue at all. None of those four variables is your undertone, which is why two people with the same undertone and differently placed veins get opposite answers from the same test.
Supports · That perceived vein colour is governed by vessel geometry and tissue optics, from a peer-reviewed optics paper devoted to the question.
Does not establish · That the test fails specifically on deep skin. The paper does not discuss melanin at all. The honest statement is that the mechanism the test assumes is not the mechanism the literature describes — plus the practical observation, which is not from this paper, that on deeper skin there is often nothing visible to read.
Applied Optics →