  We present the study of the metallicity dependence of the Fourier amplitude
ratios R_21 and R_31 for the light curves of short-period Galactic classical
Cepheids in B, V, R_C and I_C bands. Based on the available photometric and
spectroscopic data we determined the relations between the atmospheric iron
abundance, [Fe/H], and the Fourier parameters. Using these relations we
calculated the photometric [Fe/H] values of all program Cepheids with an
average accuracy of 0.15 dex. No spectroscopic iron abundance was known before
for 14 of these stars. These empirical results provide an alternate method to
determine the iron abundance of classical Cepheids too faint for spectroscopic
observations. We also checked whether the metal-poor Cepheids of both
Magellanic Clouds follow the same relationships, and a good agreement was
found.
