  In an era of extensive photometric observations, the catalogs of RR Lyr type
variable stars number tens of thousands of objects. The relation between the
iron abundance [Fe/H] and the Fourier parameters of the stars light curve
allows us to investigate mean metallicities and metallicity gradients in
various stellar environments, independently of time-consuming spectroscopic
observations. In this paper we use almost 6500 $V$- and $I$-band light curves
of fundamental mode RR Lyr stars from the OGLE-IV survey to provide a relation
between the $V$- and $I$-band phase parameter $\varphi_{31}$ used to estimate
[Fe/H]. The relation depends on metallicity, which limits its applicability. We
apply this relation to metallicity formulae developed for the Johnson $V$- and
the Kepler $Kp$-band to obtain the relation between [Fe/H] and $\varphi_{31}$
for the $I$-band photometry. Last, we apply the new relation of Nemec to the
OGLE-IV fundamental mode RR Lyr stars data and construct a metallicity map of
the Magellanic Clouds. Median [Fe/H] is $-1.39\pm0.44$ dex for the LMC and
$-1.77\pm0.48$ dex for the SMC, on the Jurcsik metallicity scale. We also find
a metallicity gradient within the LMC with a slope of $-0.029\pm0.002$ dex/kpc
in the inner 5 kpc and $-0.030 \pm0.003$ dex/kpc beyond 8 kpc, and no gradient
in-between ($-0.019\pm0.002$ dex/kpc integrally). We do not observe a
metallicity gradient in the SMC, although we show that the metal-rich RRab
stars are more concentrated toward the SMC center than the metal-poor.
