We have measured and analyzed the behavior of the nonlinear refractive index of silver spheres in water in a non-resonant femtosecond excitation regime. Two different diameter silver nanosphere (0.65 and 9 nm) suspensions were used in the experiments. Thermal and nonthermal contributions to the nonlinear properties of the samples were determined exploring a novel high sensitivity thermal managed eclipse Z-scan technique. The dependence of nonthermal third order nonlinear susceptibility of the colloid with the silver nanoparticles filling factor was described using the generalized Maxwell-Garnett model. The nanoparticle size dependence of the colloid nonlinear refractive index was observed.