Shade names are marketing; depth and undertone are the two facts that actually transfer between brands. Once you know yours, almost any shade grid becomes readable - and the mismatches you have already had are the fastest way to work them out.
Depth is how light or deep the skin is; undertone is the colour underneath - cool (pink, red, blue), warm (yellow, golden, peach) or neutral. Two shades at the same depth with different undertones look like different products on the same face. Most brand shade codes encode both, usually as a number for depth and a letter - C, N, W - for undertone.
A foundation that reads grey or ashy was too cool. One that turns orange over the day was too warm, or oxidised. One that looked right in the shop and wrong outside was matched under store lighting. Each of those points cleanly at a correction, which is why the conversation starts with what went wrong rather than what you want.
Swatch on the jawline rather than the wrist or the back of the hand - neither matches your face. Check it outside, not under shop lights. And wait ten minutes, because many formulas shift as they set. A shade that disappears on the jaw and does not reappear when you walk outdoors is the match.
What if I am between two shades?
Very common. Buy both if you can - most people need slightly different depths in summer and winter - or take the lighter one and warm it down with bronzer, which is easier than lightening a too-deep shade.
How much lighter should concealer be?
Under the eye, roughly half a shade to one shade lighter with a slightly warmer undertone to counter blue. Over blemishes, the exact match to your foundation, not lighter.
Why does my foundation oxidise?
Some formulas react with skin oils and air and deepen over an hour or two. Going one step cooler or lighter usually solves it, and the advisor will tell you which.
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