For single stellar populations (i.e. a mass of stars that forms at a single epoch with a specific mass function), the evolution of such a population is well understood and forms the basis for understanding globular clusters.

After 13 Gyrs, a SSP looks like the following diagram, with each of the interesting parts of a stars evolution marked.

SSP models allow us to calculate the change in a populations flux, color and spectrum with time. The following diagram displays the importance of various parts to a SSP's total luminosity.