The calculation of color gamuts is often based on transforming the colorant boundaries to color space. This method works for ＇＇well behaved＇＇ processes, bur in some cases it is necessary to take into account not only these physical boundaries, but also other-kinds of boundary surfaces. These additional boundaries are introduced here for 3-ink and 4-ink processes and are called natural and hybrid boundaries. Examples of print processes for which these different kinds of boundaries should be taken into account are given for the Neugebauer and localized Neugebauer model. Finally, an analytical approach is presented for the calculation of color gamuts of ＇＇well behaved＇＇ processes based on the Neugebauer model.