  We present new $H$ (1.5-1.8 $\mu$m) photometric and $K_1$ (1.9-2.2 $\mu$m)
spectroscopic observations of the young exoplanet HD 95086 b obtained with the
Gemini Planet Imager. The $H$-band magnitude has been significantly improved
relative to previous measurements, whereas the low resolution $K_1$
($\lambda/\delta\lambda \approx 66$) spectrum is featureless within the
measurement uncertainties, and presents a monotonically increasing
pseudo-continuum consistent with a cloudy atmosphere. By combining these new
measurements with literature $L^{\prime}$ photometry, we compare the spectral
energy distribution of the planet to other young planetary-mass companions,
field brown dwarfs, and to the predictions of grids of model atmospheres. HD
95086 b is over a magnitude redder in $K_1-L^{\prime}$ color than 2MASS
J12073346-3932539 b and HR 8799 c and d, despite having a similar $L^{\prime}$
magnitude. Considering only the near-infrared measurements, HD 95086 b is most
analogous to the brown dwarfs 2MASS J2244316+204343 and 2MASS
J21481633+4003594, both of which are thought to have dusty atmospheres.
Morphologically, the spectral energy distribution of HD 95086 b is best fit by
low temperature ($T_{\rm eff} =$ 800-1300 K), low surface gravity spectra from
models which simulate high photospheric dust content. This range of effective
temperatures is consistent with field L/T transition objects, but the spectral
type of HD 95086 b is poorly constrained between early L and late T due to its
unusual position the color-magnitude diagram, demonstrating the difficulty in
spectral typing young, low surface gravity substellar objects. As one of the
reddest such objects, HD 95086 b represents an important empirical benchmark
against which our current understanding of the atmospheric properties of young
extrasolar planets can be tested.
