  We investigate the UV-optical (longward of Ly$\alpha$ 1216\AA) spectral
variability of nearly 9000 quasars ($0<z<4$) using multi-epoch photometric data
within the SDSS Stripe 82 region. The regression slope in the flux-flux space
of a quasar light curve directly measures the color of the flux difference
spectrum, then the spectral shape of the flux difference spectra can be derived
by taking a careful look at the redshift dependence of the regression slopes.
First, we confirm that the observed quasar spectrum becomes bluer when the
quasar becomes brighter. We infer the spectral index of the composite
difference spectrum as $\alpha_{\nu}^{\text{dif}}\sim +1/3$ (in the form of
$f_{\nu}\propto \nu^{\alpha_{\nu}}$), which is significantly bluer than that of
the composite spectrum $\alpha_{\nu}^{\text{com}}\sim -0.5$. We also show that
the continuum variability cannot be explained by the accretion disk models with
varying mass accretion rate. Second, we examine the effects of broad emission
line variability on the color-redshift space. The variability of the "Small
Blue Bump" is extensively discussed. We show that the low-ionization lines of
MgII and FeII are less variable compared to Balmer emission lines and
high-ionization lines, and the Balmer continuum is the dominant variable source
around $\sim 3000$\AA. These results are compared with previous studies, and
the physical mechanisms of the variability of the continuum and emission lines
are discussed.
