  The UV/optical variability of active galactic nuclei and quasars is useful
for understanding the physics of the accretion disk and is gradually attributed
to the stochastic fluctuations over the accretion disk. Quasars generally
appear bluer when they brighten in the UV/optical, the nature of which remains
controversial. Recently \citeauthor{Sun2014} discovered that the color
variation of quasars is timescale dependent, in the way that faster variations
are even bluer than longer term ones. While this discovery can directly rule
out models that simply attribute the color variation to contamination from the
host galaxies, or to changes in the global accretion rates, it favors the
stochastic disk fluctuation model as fluctuations in the innermost hotter disk
could dominate the short-term variations. In this work, we show that a revised
inhomogeneous disk model, where the characteristic timescales of thermal
fluctuations in the disk are radius-dependent (i.e., $\tau \sim r$; based on
the one originally proposed by \citeauthor{DexterAgol2011}), can well reproduce
a timescale dependent color variation pattern, similar to the observed one and
unaffected by the un-even sampling and photometric error. This demonstrates
that one may statistically use variation emission at different timescales to
spatially resolve the accretion disk in quasars, thus opens a new window to
probe and test the accretion disk physics in the era of time domain astronomy.
Caveats of the current model, which ought to be addressed in future
simulations, are discussed.
