  We present a secondary eclipse observation for the hot Jupiter HD189733b
across the wavelength range 290-570nm made using the Space Telescope Imaging
Spectrograph on the Hubble Space Telescope. We measure geometric albedos of Ag
= 0.40 \pm 0.12 across 290-450nm and Ag < 0.12 across 450-570nm at 1-sigma
confidence. The albedo decrease toward longer wavelengths is also apparent when
using six wavelength bins over the same wavelength range. This can be
interpreted as evidence for optically thick reflective clouds on the dayside
hemisphere with sodium absorption suppressing the scattered light signal beyond
~450nm. Our best-fit albedo values imply that HD189733b would appear a deep
blue color at visible wavelengths.
