I went to a hat shop which had a Stratoliner in Caribou which I was able to see both in the awful fluorescent light in the shop, and at the window in natural daylight.
The colour was wonderful and complex, not "grey" or "brown" per se, but closest to "taupe" a colour midway between grey and brown. It's exactly how the Hopper hat looks in some scenes (particularly indoor shots) while in outdoors light the Hopper hat usually has an olive tinge. I'm sorry I missed seeing an actual prop hat in person, though the indoor fluorescent lighting of the museum would mean that I wouldn't know much about the hat's actual colour anyway.
(EDIT: I ended up getting a Stratoliner in Caribou which as best I can reckon is close.)
By the way, which belt are you fellows going with? S1 or S2?
S1 is a common Garrison Belt style, usually seen in 38mm (1.5 inch) in black.
S2 is a different, also common, Garrison Belt style seen in 38mm (1.5 inch) with a square buckle and 44mm (1.75 inch) with a rectangular buckle.
Hopper's appears to be the latter.
This closeup from S2 shows the belt more clearly. Things I noted 1) the leather has been aged from its original colour 2) the belt lacks scribe-lines at the borders 3) there's a crudely punched extra hole past the last original hole, as if Hopper had lost weight at some point and had to make the belt smaller.
Also note that the holster looks different, perhaps, from the S1 holster.
Update Halloween 2020: Just in time for Halloween I quickly put together the Season 2 belt.
Here's my replicas of the S1 and S2 belts: