  The observational diversity of optical emission, which coincides with prompt
gamma-ray bursts (GRBs), has been discovered in the recent Swift era. We show
that on the assumption of the synchrotron radiation for the observed energy
range below the X-ray band, the observed diversity can be explained using the
internal shock model by taking into account a high-latitude emission and the
spectral change due to the synchrotron self-absorption. It may even be possible
in our model to include bright optical flashes found, e.g., in GRB~990123. The
prediction of our model is that the spectral index in the optical band is
dependent on whether the optical light curve correlates with those in the
X-rays and/or gamma-rays or not, which will be tested in the near future.
