The final structure of a colloidal system is greatly influenced by the mechanisms by which phase separation and aggregation occur. The drive to phase separate can be altered in colloid-polymer mixtures (which phase separate due to the depletion interaction) by varying the polymer concentration. Here, we use small angle light scattering to follow the phase separation in such mixtures and analyze the results within a framework indicated by previous results from microscopy investigations. The mechanisms of diffusion-limited cluster aggregation, reaction-limited cluster aggregation, and nucleation and growth are found to provide good descriptions of the phase separation regimes. The growth rate in the nucleation and growth regime is shown to be dependent on the polymer concentration.