  The purpose of the paper is to predict the temperature at the fundamental
blue edge (FBE) of the instability strip for RR Lyrae (RRL) variables from the
pulsation equation that relates temperature to period, luminosity, and mass.
Modern data for the correlations between period, luminosity, and metallicity at
the FBE for field and cluster RRL are used for the temperature calculation. The
predicted temperatures are changed to B-V colors using an adopted color
transformation. The predicted temperatures at the FBE become hotter as [Fe/H]
changes from 0 to -1.5, and thereafter cooler as the metallicity decreases to
-2.5 and beyond. The temperature range over this interval of metallicity is
$\Delta$log $T_e$ = 0.04, or 640 K at 6900K. The predicted color variation is
at the level of 0.03 mag in B-V. The predictions are compared with the observed
RRL colors at the FBE for both the field and cluster variables, showing general
agreement at the level of 0.02 mag in (B-V)$_o$, which, however, is the
uncertainty of the reddening corrections.
  The focus of the problem is then reversed by fitting a better envelope to the
observed FBE relation between color and metallicity for metallicities smaller
than -1.8 which, when inserted in the pulsation equation, gives a non-linear
calibration ....
