Why does my foundation look orange?

Oxidation is a clock, not a mistake

Iron oxides are what most liquid foundation is coloured with, and they warm on contact with skin oil and air. That is why the shade you matched on a clean jaw at eleven is a shade warmer at three, and why it happens more on oily skin and in heat. It is not a defect and it is not universal -- formulas differ a great deal -- but it is the reason a foundation can be right in the shop and wrong by dinner. Test a sample across a whole day before buying the bottle.

Undertone is a different axis from depth

Depth is how light or dark a shade is; undertone is whether it leans warm, neutral or cool. Get the depth right and the undertone wrong and the result reads orange, or grey, or simply like makeup. Most ranges label undertone somewhere -- W, N and C, or golden, neutral and rosy -- and it is the letter rather than the number that decides whether a shade disappears.

Test on the jaw, in daylight, and wait

The inside of the wrist is a different colour from the face and the hand is different again, so a swatch there proves nothing. Draw a stripe along the jaw, go outside, and look. The right shade is the one you cannot find. Shop lighting is warm and even, which flatters every shade and is why so many bottles are bought that never get used.

Grey rather than orange is usually the other direction

A base that reads ashy or grey is generally too cool for the skin under it, and on deeper skin that is the commonest mismatch of all -- ranges tend to be built pink at the light end and grey at the deep end. Flash photography adds a separate problem: some mineral sunscreens bounce light back and produce a pale cast that is invisible in the room and obvious in the picture.

Nothing on this shelf corrects a base that has already turned

Said plainly, because the honest answer to this question is mostly not a purchase. There is no colour corrector here and no shade-adjusting drop, and a concealer will not do that job either -- a concealer has an undertone of its own, and a lighter one under the eye against a warm base is one of the ways a face reads orange in a photograph rather than a cure for it. The two products below earn their place narrowly: a moderate-coverage foundation puts less pigment on the face to warm up, and a translucent powder sets without voting on a shade. The fix for an orange base is the shade you buy next.

The neck is the tell, not the face

A face and a neck are rarely the same colour, and the honest answer is to match the neck rather than the face -- the jawline is where a mismatch shows and the neck is what the world sees beside it. If the two are far apart, going a shade lighter and letting the skin do the rest reads better than trying to bring the neck up to meet the face.

Questions

Can I fix an orange foundation I already own?

A little, and not reliably. Mixing in a drop of a cooler or lighter base of the same finish evens it out; colour-correcting drops do the same job more precisely. What does not work is powdering over it, because a warm base under a translucent powder is still warm.

Does foundation oxidise on everyone?

No, and it is not a fixed property of the person either. It depends on the formula, on how much oil the skin produces and on the day. Somebody with dry skin may never see it in a formula that turns visibly on somebody else.

Should I go a shade lighter to be safe?

Not as a rule. A shade too light reads grey or chalky rather than natural, and it is the more obvious error in daylight. Match the neck, and if the formula is one that warms, account for that rather than starting pale.