  We report a detection of statistically significant color variations for a
sample of 7,531 multiply observed asteroids that are listed in the Sloan
Digital Sky Survey Moving Object Catalog. Using 5-band photometric observations
accurate to ~0.02 mag, we detect color variations in the range 0.06-0.11 mag
(rms). These variations appear uncorrelated with asteroids physical
characteristics such as diameter (in the probed 1-10 km range), taxonomic
class, and family membership. Despite such a lack of correlation, which implies
a random nature for the cause of color variability, a suite of tests suggest
that the detected variations are not instrumental effects. In particular, the
observed color variations are incompatible with photometric errors, and, for
objects observed at least four times, the color change in the first pair of
observations is correlated with the color change in the second pair. These
facts strongly suggest that the observed effect is real, and also indicate that
for some asteroids color variations are larger than for others. The detected
color variations can be explained as due to inhomogeneous albedo distribution
over an asteroid surface. Although relatively small, these variations suggest
that fairly large patches with different color than their surroundings exist on
a significant fraction of asteroids. This conclusion is in agreement with
spatially resolved color images of several large asteroids obtained by NEAR
spacecraft and HST.
