We analyze 2–5 \mu m spectroscopic observations of the dust coma of comet 67P/Churyumov-Gerasimenko obtained with the VIRTIS-H instrument onboard Rosetta from 3 June to 29 October 2015 at heliocentric distances r _ { h } = 1.24–1.55 AU .
The 2–2.5 \mu m color , bolometric albedo , and color temperature are measured using spectral fitting .
Data obtained at \alpha = 90 ^ { \circ } solar phase angle show an increase of the bolometric albedo ( 0.05 to 0.14 ) with increasing altitude ( 0.5 to 8 km ) , accompanied by a possible marginal decrease of the color and color temperature .
Possible explanations include the presence in the inner coma of dark particles on ballistic trajectories , and radial changes in particle composition .
In the phase angle range 50–120 ^ { \circ } , phase reddening is significant ( 0.031 % /100 nm/ ^ { \circ } ) , for a mean color of 2 % /100 nm at \alpha = 90 ^ { \circ } , that can be related to the roughness of the dust particles .
Moreover , a decrease of the color temperature with decreasing phase angle is also observed at a rate of \sim 0.3 K/ ^ { \circ } , consistent with the presence of large porous particles , with low thermal inertia , and showing a significant day-to-night temperature contrast .
Comparing data acquired at fixed phase angle ( \alpha = 90 ^ { \circ } ) , a 20 % increase of the bolometric albedo is observed near perihelion .
Heliocentric variations of the dust color are not significant in the analyzed time period .
Measured color temperatures are varying from 260 to 320 K , and follow a r _ { h } ^ { 0.6 } variation in the r _ { h } = 1.24–1.5 AU range , close to the expected r _ { h } ^ { 0.5 } value .