Foundation has two variables. Depth is how light or deep the shade is, and most people judge it correctly by eye. Undertone is the colour underneath — warm, cool, or neutral — and it is where a shade that looked right in the shop turns grey or orange by lunchtime.
Three quick tests
Look at the veins on the inside of your wrist in daylight: greenish suggests warm, blue or purple suggests cool, and a mix suggests neutral. Consider which metal flatters you more — gold tends to suit warm, silver cool. And think about how you burn: skin that tans easily often skews warm.
None of these is definitive on its own. Taken together they usually agree, and that agreement is your answer.
Swatch on the jaw, not the hand
The back of the hand is frequently a different depth from the face entirely. Swatch three candidate shades along the jawline, then check them in daylight — not under shop lighting, which is engineered to flatter. The correct shade disappears.
Buy for two seasons
Most people shift by roughly half a shade between summer and winter. Two foundations, or one foundation plus a drop of a lighter or deeper adjuster, covers the year. Mixing your own in-between shade is normal and costs nothing.
Oxidation is real
Some formulas darken slightly on the skin as they react with air and sebum. Give a swatch ten minutes before deciding. A shade that matches instantly but looks orange after fifteen minutes was never the right one.