  We introduce a general physical formulation that allows one to obtain
uniquely the effectiveinteractions by contracting the bare forces, even in
highly concentrated systems. We tested it bystudying depletion forces in
two-dimensional binary and ternary colloidal mixtures with a totalpacking
fraction up to 70%. Our result opens up the possibility of finding an efficient
route todetermine effective interactions at finite concentration and even at
thermodynamic conditions nearto meta-stable or out of equilibrium states.
