Simulations of charged colloidal dispersions are technically challenging. One possible workaround consists in reducing the system to the colloids only, whose interactions are described through an effective pair potential, w^f. Still, the determination of w^f is difficult mainly because it depends on the colloidal density, ϕ. Here we propose to calculate w^f from simulations of a pair of colloids placed in a cubic box with periodic boundary conditions. The variation in ϕ is mimicked by an appropriate change in the concentration of counterions neutralized by an homogeneous background charge. The method is tested at the level of the primitive model. A good description of the structure of the colloidal dispersion is obtained in the low and high coupling regimes, even at high ϕ (≈30%). Furthermore, the method can easily be used in popular molecular simulation program packages and extended to non-spherical objects.