Said method comprises a first cusp-oriented lightness-mapping step and a second lightness-mapping step in which a source color (N) is mapped into a mapped color (N″) having the same hue (H″=H) and the same chroma (C″=C) as the source color, where the second mapping step is defined such as to bring more colors in a target color gamut and more colors closer to the boundary of this target gamut.