  Among the greatest challenges in understanding ultra-cool brown dwarf and
exoplanet atmospheres is the evolution of cloud structure as a function of
temperature and gravity. In this study, we present the rotational modulations
of GU Psc b -- a rare mid-T spectral type planetary-mass companion at the end
of the L/T spectral type transition. Based on the HST/WFC3 1.1-1.67$\rm\, \mu
m$ time-series spectra, we observe a quasi-sinusoidal light curve with a
peak-to-trough flux variation of 2.7 % and a minimum period of eight hours. The
rotation-modulated spectral variations are weakly wavelength-dependent, or
largely gray between 1.1-1.67$\rm\,\mu$m. The gray modulations indicate that
heterogeneous clouds are present in the photosphere of this low-gravity mid-T
dwarf. We place the color and brightness variations of GU Psc b in the context
of rotational modulations reported for mid-L to late-T dwarfs. Based on these
observations, we report a tentative trend: mid-to-late T dwarfs become slightly
redder in $J-H$ color with increasing $J$-band brightness, while L dwarfs
become slightly bluer with increasing brightness. If this trend is verified
with more T-dwarf samples, it suggests that in addition to the mostly gray
modulations, there is a second-order spectral-type dependence on the nature of
rotational modulations.
