What we really want to measure is the effects of star formation and
chemical evolution on the integrated colors, i.e. the position of the RGB
(metallicity) and the turnoff point (age).|
While this can be complicated by broad RGB's due to a mix of stars with range of metallicities or extended upper MS due to recent star formation (see HIPPARCHUS results for the disk of the Milky Way to left), progress can be made by restricting our studies to objects with (hopefully) simple star formation histories, i.e. ellipticals and S0's.
Careful choice of the filters (i.e. regions of the SED to examine) are also important in this endeavor.