  Measurements of photometric variability at different wavelengths provide
insights into the vertical cloud structure of brown dwarfs and planetary-mass
objects. In seven Hubble Space Telescope consecutive orbits, spanning $\sim$10
h of observing time}, we obtained time-resolved spectroscopy of the
planetary-mass T8-dwarf Ross 458C using the near-infrared Wide Field Camera 3.
We found spectrophotometric variability with a peak-to-peak signal of
2.62$\pm$0.02 % (in the 1.10-1.60~$\mu$m white light curve). Using three
different methods, we estimated a rotational period of 6.75$\pm$1.58~h for the
white light curve, and similar periods for narrow $J$- and $H$- band light
curves. Sine wave fits to the narrow $J$- and $H$-band light curves suggest a
tentative phase shift between the light curves with wavelength when we allow
different periods between both light curves. If confirmed, this phase shift may
be similar to the phase shift detected earlier for the T6.5 spectral type 2MASS
J22282889-310262. We find that, in contrast with 2M2228, the variability of
Ross~458C shows evidence for a {color trend} within the narrow $J$-band, but
gray variations in the narrow $H$-band. The spectral time-resolved variability
of Ross 458C might be potentially due to heterogeneous sulfide clouds in the
atmosphere of the object. Our discovery extends the study of spectral
modulations of condensate clouds to the coolest T dwarfs, planetary-mass
companions.
