  We report on the correlation between the flux, color and polarization
variations on time scales of days--months in blazars, and discuss their
universal aspects. We performed monitoring of 42 blazars in the optical and
near-infrared bands from 2008 to 2010 using TRISPEC attached to the "Kanata"
1.5-m telescope. We found that 28 blazars exhibited "bluer-when-brighter"
trends in their whole or a part of time-series data sets. This corresponds to
88% of objects that were observed for >10 days. Thus, our observation
unambiguously confirmed that the "bluer-when-brighter" trend is common in the
emission from blazar jets. This trend was apparently generated by a variation
component with a constant and relatively blue color and an underlying red
component. Prominent short-term flares on time scales of days--weeks tended to
exhibit a spectral hysteresis; their rising phases were bluer than their decay
phases around the flare maxima. In contrast to the strong flux--color
correlation, the correlation of the flux and polarization degree was relatively
weak; only 10 objects showed significant positive correlations. Rotations of
polarization were detected only in three objects: PKS 1510-089, 3C 454.3, and
PKS 1749+096, and possibly in S5 0716+714. We also investigated the dependence
of the degree of variability on the luminosity and the synchrotron peak
frequency, \nu_peak. As a result, we found that lower luminosity and higher
\nu_peak objects had smaller variations in their amplitudes both in the flux,
color, and polarization degree. Our observation suggests the presence of
several distinct emitting sources, which have different variation time-scales,
colors, and polarizations. We propose that the energy injection by, for
example, internal shocks in relativistic shells is a major factor for blazar
variations on time scales of both days and months.
