An Estee Lauder rep once told me that if a colour looks good on your fingertip pad, it will look good on the face, and she was right! Try a little on your index finger and you can immediately tell if it will look right, plus, this saves cleaning up the lips and starting again.
To make a broad choice a little narrower though, if a client has a warm skin tone - tanned, with blue or green eyes - I stick to shades with orange or pink undertones but if a client has a cool colouring - pale, with dark brown eyes - I stick to shades with clear red or blue undertones. Black skins suit dominant, bright colours. Don't be afraid to experiment and if in doubt, ask your client which out of two or three she would prefer.