  The well-known bluer-when-brighter trend observed in quasar variability is a
signature of the complex processes in the accretion disk, and can be a probe of
the quasar variability mechanism. Using a sample of 604 variable quasars with
repeat spectra in SDSS-I/II, we construct difference spectra to investigate the
physical causes of this bluer-when-brighter trend. The continuum of our
composite difference spectrum is well-fit by a power-law, with a spectral index
in excellent agreement with previous results. We measure the spectral
variability relative to the underlying spectra of the quasars, which is
independent of any extinction, and compare to model predictions. We show that
our SDSS spectral variability results cannot be produced by global accretion
rate fluctuations in a thin disk alone. However, we find that a simple model of
a inhomogeneous disk with localized temperature fluctuations will produce
power-law spectral variability over optical wavelengths. We show that the
inhomogeneous disk will provide good fits to our observed spectral variability
if the disk has large temperature fluctuations in many independently varying
zones, in excellent agreement with independent constraints from quasar
microlensing disk sizes, their strong UV spectral continuum, and single-band
variability amplitudes. Our results provide an independent constraint on quasar
variability models, and add to the mounting evidence that quasar accretion
disks have large localized temperature fluctuations.
