All photometric or spectroscopic systems suffer from various degrees of metallicity/age
degeneracy, the problem of separating the effects of age on a integrated stellar population
(bluer = younger) from metallicity effects (bluer = metal-poor).|
For example, it is easy to distinguish SSP's (simple stellar populations, i.e. single
generation of coeval and chemically homogenous stars of various masses that
produce a unique CM diagram) of two different metallicities (see
above plots from Bruzual and Charlot SED models).