  In this paper, we investigate the color changes of gravitational microlensing
events caused by the two different mechanisms of differential amplification for
a limb-darkened extended source and blending. From this investigation, we find
that the color changes of limb-darkened extended source events (color curves)
have dramatically different characteristics depending on whether the lens
transits the source star or not. We show that for a source transit event, the
lens proper motion can be determined by simply measuring the turning time of
the color curve instead of fitting the overall color or light curves. We also
find that even for a very small fraction of blended light, the color changes
induced by the blending effect is equivalent to those caused by the
limb-darkening effect, causing serious distortion in the observed color curve.
Therefore, to obtain useful information about the lens and source star from the
color curve of a limb-darkened extended source event, it will be essential to
eliminate or correct for the blending effect. We discuss about the methods for
the efficient correction of the blending effect.
