Separate depth from undertone
Two foundation shades can look equally light or deep in a bottle yet read differently on skin. The difference may come from undertone. Instead of choosing by a label such as “warm” alone, compare neighboring shades on the jaw where you can see the face and neck together.
Let each swatch dry according to the product’s directions and check it in daylight as well as indoor light. Store lighting is designed to sell products, not necessarily to mimic your everyday setting.
Consider the finish
A matte finish and a luminous finish can make the same color seem different. Apply a small amount the way you would normally wear it, then look again after it has settled. If a shade is close but the formula feels wrong, a different product may solve more than another shade number.
Use a sample when available
A take-home sample lets you see the color through a normal day, alongside sunscreen and the rest of your makeup. That test is especially useful when purchasing online. Keep the product name and exact shade together in a note so a successful match is easy to find again.