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).