How to find your foundation shade without buying four of them

A foundation shade looks like one decision and is really three. Depth is how light or dark it is. Undertone is the colour underneath — cool, warm, neutral or olive. Oxidation is what it becomes twenty minutes after it meets your skin, which is rarely what it looked like in the bottle.

Most bad matches get depth right and one of the other two wrong. That is why a shade can be exactly the correct darkness and still read orange, grey or chalky.

Test on the jaw, never on the hand

The back of your hand is not the colour of your face. It sees more sun, it has a different texture, and it is usually a shade or two off. Your wrist is worse: it is paler than everything it is attached to.

Swipe three candidate shades side by side along your jawline, running down onto the neck. The right one disappears. If you can see where it starts and stops, it is wrong — and the direction it is wrong in tells you which of the three decisions to change.

• Too light or too dark, but the same colour family — change depth
• Orange, or too yellow — the undertone is too warm
• Grey, ashy or pink — the undertone is too cool
• Right at first, wrong after twenty minutes — oxidation

Daylight, and nothing else

Test near a window, facing it. Shop lighting is built to sell and skews warm; a bathroom bulb skews the other way. Any shade judged under a spotlight will be judged again, differently, the first time you walk outside.

Bare skin, too. Testing over yesterday’s base tells you how the new foundation looks on the old one.

Wait twenty minutes

Foundation oxidises: it reacts with the oil on your skin and with air, and most formulas go slightly darker and slightly warmer as they settle. A shade chosen in the first thirty seconds is chosen before the product has finished deciding what colour it is.

Put the swatch on, leave the shop, and look again on the way home. This single habit prevents more wrong bottles than any chart.

Why the number is different at every counter

There is no shared scale. One brand’s numbering runs light to dark; another’s runs by undertone family; a third names them after places. “220” means nothing across a counter. What travels between brands is the description — medium depth, neutral-warm undertone — and that is what to carry with you, not a number.

You probably need two

Skin moves through the year. Most people are half a shade to a full shade deeper in late summer than in February, and the undertone does not move with it. Two bottles of the same formula in adjacent depths, mixed as the year goes on, will match better than one bottle chosen as a compromise.

What Boirelle does with it

The app reads your undertone from a selfie — the decision that is hardest to make in a mirror and easiest to get wrong at a counter — along with hydration, oil, texture and redness, which decide whether a foundation should be sheer, matte or dewy on you today. The try-on studio then lays shades over your own scan, so you are looking at a colour on your face rather than on a screen.

It reads a photograph. It cannot see how a formula will oxidise on you, and it says so: where the light is wrong or the reading uncertain, it prints a dash rather than a guess.

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