The average stellar metallicity can be obtained from (V-I)-3.5,
which is the colour index of the RGB half a magnitude below the
TRGB (see Da Costa & Armandroff 1990;
Lee, Freedman, & Madore 1993).
In our case, as we have said before, it is not clear whether the colour
of the NGC 6789 RGB locus extends.
The different colour function of V-I, with different
smoothing, gives either two clearly separated peaks (a dominant number of
stars place between 
) or one solid peak with border
. Using this colour intervals, the two possible
estimates are (V-I)-3.5=1.56 and (V-I)-3.5=1.73.
(the magnitude of the TRGB is ITRGB=22.7; see
§3.3).
The resulting dereddened values for the colour index of the RGB are (V-I)-3.5,0=1.47 and and (V-I)-3.5,0=1.64. Using the calibration by Lee et al. (1993), we obtain a metallicity [Fe/H]=-1.25, or [Fe/H]=-0.85.