Our interest goes to the different virial contributions to the equation of
state of charged colloidal suspensions. Neglect of surface effects in the
computation of the colloidal virial term leads to spurious and paradoxical
results. This pitfall is one of the several facets of the danger of a naive
implementation of the so called One Component Model, where the micro-ionic
degrees of freedom are integrated out to only keep in the description the
mesoscopic (colloidal) degrees of freedom. On the other hand, due incorporation
of wall induced forces dissolves the paradox brought forth in the naive
approach, provides a consistent description, and confirms that for salt-free
systems, the colloidal contribution to the pressure is dominated by the
micro-ionic one. Much emphasis is put on the no salt case but the situation
with added electrolyte is also discussed
