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The byCa filter system was used to obtain hk and b-y colors for a dozen RR Lyr stars at various phases, where hk = (Ca - b ) - (b - y ). When the data are plotted in an hk /(b-y ) color-magnitude diagram there is a clear stratification of the data points by metallicity, with the lower metallicity stars having a lower value of hk . In the color range 0.10<b-y <0.25 the metallicity depends on hk only. The empirical relation is determined to be [Fe/H] = 12.5(hk - 0.40). The hk index is shown to be substantially more sensitive to metallicity than the Stromgren $m_1$ index for these stars, confirming previous predictions. Because hk has already been shown to be more sensitive to metallicity than $m_1$ for cooler stars, the hk index has now been shown to be quite sensitive to metallicity over a substantial range of temperature.
