For F,G,K stars, their photometric data and metallicity, [Fe/H] are collected and their correlations between δ δ (U-B) and [Fe/H] for each spectral group are examined, using a reference sequence which is defined by the stars in Woolley's catalogue and other reference sequences. Detailed examination shows that the reference sequence by Woolley's catalogue appears to be properly defined for the calculation of δ δ (U-B) of population I stars in the solar neighborhood. It is suggested that accuracies of metallicities derived from δ δ (U-B) can be improved if we derive the correlations between [Fe/H] and δ δ (U-B) from the stars in each spectral group.