We investigated cluster formation in model colloids where the interparticle potential beyond the collision diameter includes a short-ranged attraction and a longer-ranged repulsion. The structure and thermodynamic properties of the model system can be accurately described by the first-order mean-spherical approximation (FMSA) and a perturbation theory for bulk phases and by a nonlocal density-functional theory (DFT) for stable/metastable clusters. In corroboration with recent experiments and molecular simulations, we predicted that the bulk to cluster transition may occur at both one-phase and two-phase regions of the colloidal phase diagram. While formation of colloidal clusters appears uncorrelated with the bulk phase transitions, the local phenomena may nevertheless play an important role in the dynamics of stable colloids and the kinetics of colloidal phase transitions.