Brown? why would you want it in brown?
other than the rust that might have appeared on the panels, Willie was assumed to have been painted gray.
The current Willie is painted more a "battleship gray".
Bovington Tank Museum's David Fletcher said to me "a darker Royal Navy gray".
The original shade of gray isn't known.
I still contend, in the photographs where the final version of Willie was presented to the press and Public at the Fosters Factory later in 1915,
it had been given a fresh coat of paint...and I thought maybe it was green. (below)
But since B/W photography can convert colour and does not reveal hints of true colours, Willie could have been painted bright yellow!
But according to Tank expert David Fletcher MBE,
early Mark 1 tanks were painted gray when they came out of the factory
and he has no other evidence of paint colour on Willie at that time.
Willie was painted green by the Bovington Tank museum in the 1960s, but he says thats because
"the museum painted everything green, since they had a surplus of green paint".
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Sepia toning (reddish-brown tinting) in B/W photography may have been used more for Portraiture and Life scene photos
where a warmer hue was desired.
The second photo of Willie (above) has a more "sepia" tone
but this photo can be found in various tones and shades (across the Internet).
Obviously more recent digital toning is responsible for that.