Contrast

Contrast is mostly a matter of depth. A deep plum on fair skin, or a pale pink on deep skin, creates high contrast: the mouth becomes the focal point and the shape of the lips is clearly drawn. A shade close to the skin’s own depth creates low contrast and reads as soft or natural.

Colour adds to it. A vivid shade with high chroma stands out more than a muted one at the same lightness, and a shade whose undertone is the opposite of the skin’s is more noticeable than a matching one.

Neither high nor low contrast is better; they give different looks. The skin-depth hubs on this site use contrast as their guide when listing shades. The nude lists go further and check how close each shade sits to typical skin lightness, because a nude much lighter than the skin can read like concealer rather than lip colour.