A grand canonical Monte Carlo procedure is used to simulate some mechanical properties (pressure, shear modulus) of charged colloidal spheres immersed in a solution of electrolyte. Particular attention is devoted to the detection of attractive correlation forces that could be responsible for discrepancies in diffuse layer theory. Although a high value of the shear modulus is reported for semidilute suspensions, the equation of state of the colloidal suspension agrees well with the prediction of the diffuse layer model.