A method based on the random phase approximation (RPA) is used to calculate the effective interparticle pair potential ϕ(r) in a concentrated charged colloidal dispersion from an observed structure factorS(q). By using Monte Carlo simulation results for inverse power potential fluids at their melting points as reference systems, the perturbation scheme determines ϕ(r) at the melting point of the colloid from experimental data. The resulting ϕ(r) is found to be oscillatory and differ qualitatively from the form predicted by DLVO theory.