  The spectral signatures of optical absorbers, when combined with those of
infrared molecules, play a critical role in constraining the cloud properties
of exoplanet atmospheres. We aim to use optical transmission spectroscopy to
confirm the tentative color signature previously observed by multiband
photometry in the atmosphere of hot Jupiter HAT-P-55b. We observed a transit of
HAT-P-55b with the OSIRIS spectrograph on the Gran Telescopio Canarias (GTC).
We created two sets of spectroscopic light curves using the conventional
band-integrated method and the newly proposed pixel-based method to derive the
transmission spectrum. We performed Bayesian spectral retrieval analyses on the
transmission spectrum to interpret the observed atmospheric properties. The
transmission spectra derived from the two methods are consistent, both
spectrally resolving the tentative color signature observed by MuSCAT2. The
retrievals on the combined OSIRIS and MuSCAT2 transmission spectrum yield the
detection of Na at 5.5$\sigma$ and the tentative detection of MgH at
3.4$\sigma$. The current optical-only wavelength coverage cannot constrain the
absolute abundances of the atmospheric species. Space-based observations
covering the molecular infrared bands or ground-based high-resolution
spectroscopy are needed to further constrain the atmospheric properties of
HAT-P-55b.
