Colloids suspended in a binary solvent may, under suitable thermodynamic conditions, experience a wide variety of solvent-mediated interactions that can lead to colloidal phase transitions and aggregation phenomena. We present a simple mean-field theory, based on free-volume arguments, that describes the phase behaviour of colloids suspended in a near-critical binary solvent. The theory predicts rich phase behaviour: we find colloidal gas, liquid and crystal phases, a colloidal gas–liquid critical line and a colloidal solid–solid critical line. We compare our results with those of our recent simulation study of the same model in two dimensions. Our simple theory accounts for the main features of the phase diagrams found in simulations and sheds new light on the origin of colloidal aggregation lines in near-critical solvents.