  We present a JavaScript calculator which provides the redshift evolution of
the luminosities and colors of passively evolving galaxies for a range of
rest-frame filters. In its simplest form it gives the evolution of a single age
stellar population formed at a given redshift. However, it can also incorporate
morphological evolution in the galaxy population ("progenitor bias") and
complex star formation histories of individual galaxies. The calculator is
based on the models of van Dokkum & Franx (2001), with small updates.
