Although critical in applications, the dynamics of colloidal systems at low Péclet numbers is poorly understood. Here we introduce an optical technique that permits for the first time a complete characterization of this regime through a continuous and independent measurement of both the diffusive and the advective components of a system's dynamics. For the particular example of gravity-driven colloids, we demonstrate experimentally that the hydrodynamic size and the mass density of particulate suspensions can be measured simultaneously. The proven capabilities are of particular interest for studying the spatial and temporal properties of inhomogeneous colloidal systems where aggregation and structural evolution play major roles.