  Using archival Hubble Space Telescope (HST) imaging data, we report the
multiband photometric properties of 13 ultraluminous X-ray sources (ULXs) that
have a unique compact optical counterpart. Both magnitude and color variation
are detected at time scales of days to years. The optical color, variability,
and X-ray to optical flux ratio indicate that the optical emission of most ULXs
is dominated by X-ray reprocessing on the disk, similar to that of low mass
X-ray binaries. For most sources, the optical spectrum is a power-law, $F_{\nu}
\propto \nu^{\alpha}$ with $\alpha$ in the range 1.0 to 2.0 and the optically
emitting region has a size on the order of 1e12 cm. Exceptions are NGC 2403 X-1
and M83 IXO 82, which show optical spectra consistent with direct emission from
a standard thin disk, M101 ULX-1 and M81 ULS1, which have X-ray to optical flux
ratios more similar to high-mass X-ray binaries, and IC 342 X-1, in which the
optical light may be dominated by the companion star. Inconsistent extinction
between the optical counterpart of NGC 5204 X-1 and the nearby optical nebulae
suggests that they may be unrelated.
