  Context. Although a bluer-when-brighter trend is commonly observed in
blazars, the opposite trend of redder-when-brighter has also been found in some
blazars. Aims. We investigate the frequency of the redder-when-brighter trend
in flat-spectrum radio quasars (FSRQs). Methods. We investigate the optical
variability of 29 FSRQs in the SDSS Stripe 82 region using SDSS DR7 released
multi-epoch data covering about nine years. We determined the spectral index by
fitting a powerlaw to SDSS ugriz photometric data, and explored the
relationship between the spectral index and source brightness. Results. For all
FSRQs studied, we detect variations in r band flux of overall amplitude between
0.24 mag and 3.46 mag in different sources. Fourteen of 29 FSRQs display a
bluer-when-brighter trend. However, only one source exhibits a
redder-when-brighter trend, which implies that this behavior is rare in our
FSRQ sample. In this source, the thermal emission from the accretion disk may
be responsible for the redder-when-brighter trend.
