We describe a charge-stabilized colloidal suspension within a Poisson-Boltzmann cell model and calculate the free energy as well as the compressibility as a function of colloidal density. The same quantities are also calculated from the linearized Poisson-Boltzmann equation. Comparing nonlinear with linear theory, we test the quality of different linearization schemes. For concentrated suspensions, linearization about the Donnan potential is shown to be preferable to standard Debye-Hückel linearization. We also show that the volume term theory proposed earlier follows from a linearization about the Donnan potential. Using this linearization scheme, we find a gas-liquid phase coexistence in linear, but not in nonlinear theory. This result may imply that predictions of a spinodal instability in highly de-ionized colloidal suspensions are spurious.