  We present a numerical study of the effective potential $V_\mathrm{eff}$
between two hard-sphere colloids dispersed in a solvent of interacting
particles, for several values of temperature and solvent density, approaching
the solvent gas-liquid critical point. We investigate the stability of a system
of particles interacting via $V_\mathrm{eff}$ to evaluate the locus of
colloidal aggregation in the solvent phase-diagram, and its dependence on the
colloid size. We assess how the excluded volume depletion forces are modified
by solvent attraction and discuss under which conditions solvent critical
fluctuations, in the form of critical Casimir forces, can be used to
effectively manipulate colloidal aggregation.
