The effective interaction between colloids in a solvent of hard spheres, which can polymerize with the formation of cross-like molecular clusters, is studied on the basis of the solution of the associative Percus-Yevick/Ornstein-Zernike equation for spatial correlations in an infinitely dilute solution. It is found that at low solvent density and low temperature, the effective intermolecular potential between the colloidal particles is attractive, thus facilitating a phase separation or precipitation of the colloids. A depletion of the local solvent density around the colloidal particle is observed. If the solvent consists of extended associative complexes, compared with hard spheres, the correlation between the colloid particles are of longer range and barrier for flocculation shifts to the larger separations. The effect of size ratio of the colloid spheres and of solvent on the potential of mean force is also discussed.