  We present UBVI photometry for 8530 stars in Baade's Window obtained during
the OGLE-II microlensing survey. Among these are over one thousand red clump
giants. 1391 of them have photometry with errors smaller than 0.04, 0.06, 0.12,
and 0.20 mag in the I, V, B and U-band, respectively. We constructed a map of
interstellar reddening. The corrected colors of the red clump giants: (U-B)_0,
(B-V)_0, and (V-I)_0 are very well correlated, indicating that a single
parameter determines the observed spread of their values, reaching almost 2 mag
in the (U-B)_0. It seems most likely that heavy element content is the dominant
parameter, but it is possible that another parameter: the age (or mass) of a
star moves it along the same trajectory in the color--color diagram as the
metallicity. The current ambiguity can be resolved with spectral analysis, and
our catalog may be useful as a finding list of red clump giants. We point out
that these K giants are more suitable for a fair determination of the
distribution of metallicity than brighter M giants.
  We also present a compilation of UBVI data for 308 red clump giants near the
Sun, for which Hipparcos parallaxes are more accurate than 10%. Spectral
analysis of their metallicity may provide information about the local
metallicity distribution as well as the extent to which mass (age) of these
stars affects their colors.
  It is remarkable that in spite of a number of problems, stellar models agree
with observations at the 0.1-0.2 mag level, making red clump giants not only
the best calibrated but also the best understood standard candle.
