Tunable two-color ultrafast pump–probe experiments are performed on colloidal metal nanospheres in water and ethanol, and the results are interpreted through a model including thermal dynamics in both the nanoparticle and its environment. Transient optical signals measured for gold nanoparticles in ethanol display a complex temporal response strongly dependent on the wavelength of the probe beam. We demonstrate that its origin is related to a large contribution of liquid environment transient heating to the optical response, in contrast to the usual assumption that optical transmission changes reflect the evolution of nanoparticle temperature only. Accounting for this contribution enables an accurate measurement of thermal conductances at gold–ethanol and gold–water interfaces. Moreover, the thermal dynamics of the solvent can be selectively probed through a specific choice of probe wavelength.