  The addition of co-solutes to colloidal suspensions is often employed to
induce tunable depletion interactions. In this work we investigate effective
colloidal interactions arising from binary co-solute mixtures of hard spheres
and patchy particles. By changing the relative concentration of the two
species, we show that the resulting effective potential $V_\text{eff}$
continuously changes from the one obtained for a single-component hard sphere
co-solute to that mediated by the single-component patchy particle co-solute.
Interestingly, we find that, independently of the relative concentration of the
two components, the resulting $V_\text{eff}$ is additive, i.e., it is
well-described by the linear combination of the effective interactions mediated
by respective pure co-solutes. However, a breakdown of the additivity occurs
when the co-solute mixture is close to the onset of a demixing transition.
These results represent a step forward in understanding and predicting
colloidal behaviour in complex and crowded environments and for exploiting this
knowledge to design targeted colloidal superstructures.
