For a flawless foundation match, you need to know what your skin's undertone is. Regardless of your skin's surface tone, you can end up with an ashy, muddy or dull complexion if the undertone is wrong.
There are three basic categories: cool undertones are pink, red or blue. Warm are yellow to olive, while some people have neutral undertones.
A quick rule of thumb is to check your wrist. If the veins are blue or purple, you have a cool undertone, if they appear green, you're in the warm-toned spectrum.
Once you know your undertone, match a foundation closest to your surface skin colour - the right shade for your complexion will blend in with no demarcation line at the jaw.