  We extend the colour calibration of the widely used BaSeL standard stellar
library (Lejeune, Cuisinier, & Buser 1997, 1998) to non-solar metallicities,
down to [Fe/H] ~ -2.0 dex. Surprisingly, we find that at the present epoch it
is virtually impossible to establish a unique calibration of UBVRIJHKL colours
in terms of stellar metallicity [Fe/H] which is consistent simultaneously with
both colour-temperature relations and colour-absolute magnitude diagrams (CMDs)
based on observed globular cluster photometry data and on published, currently
popular standard stellar evolutionary tracks and isochrones. The problem
appears to be related to the long-standing incompleteness in our understanding
of convection in late-type stellar evolution, but is also due to a serious lack
of relevant observational calibration data that would help resolve, or at least
further significant progress towards resolving this issue. In view of the most
important applications of the BaSeL library, we here propose two different
metallicity calibration versions: (1) the "WLBC 99" library, which consistently
matches empirical colour-temperature relations and which, therefore, should
make an ideal tool for the study of individual stars; and (2), the "PADOVA
2000" library, which provides isochrones from the Padova 2000 grid (Girardi et
al., 2000) that successfully reproduce Galactic globular-cluster
colour-absolute magnitude diagrams and which thus should prove particularly
useful for studies of collective phenomena in stellar populations in clusters
and galaxies.
