The optical variability of SDSS quasars from multi-epoch spectroscopy.
  II. color variation
  We investigated the optical/ultraviolet (UV) color variations for a sample of
2169 quasars based on multi-epoch spectroscopy in the Sloan Digital Sky Survey
(SDSS) data release seven (DR7) and data release nine (DR9). To correct the
systematic difference between DR7 and DR9 due to the different instrumental
setup, we produced a correction spectrum by using a sample of F-stars observed
both in DR7 and DR9. The correction spectrum was then applied to quasars when
comparing the spectra of DR7 with DR9. In each object, the color variation was
explored by comparing the spectral index of the continuum power-law fit on the
brightest spectrum with the faintest one, and also by the shape of their
difference spectrum. In 1876 quasars with consistent color variations from two
methods, we found that most sources (1755, $\sim 94\%$) show
bluer-when-brighter (BWB) trend, and the redder-when-brighter (RWB) trend is
only detected in 121 objects ($\sim 6\%$). The common BWB trend is supported by
the bluer composite spectrum constructed from bright spectra than that from
faint spectra, and also by the blue composite difference spectrum. The
correction spectrum is proved to be highly reliable by comparing the composite
spectrum from corrected DR9 and original DR7 spectra. Assuming that the
optical/UV variability is triggered by fluctuations, RWB trend can likely be
explained if the fluctuations occur firstly at outer disk region, and the inner
disk region has not fully responded yet when the fluctuation being propagated
inward. In contrast, the common BWB trend implies that the fluctuations are
likely more often happening firstly in inner disk region.
