Mike Snook has this:
Contemporaneous imagery suggests that, at the time of the Mutiny, the woodwork of HEIC gun carriages was painted grey, while gun barrels and metal fittings were black. There are also surviving bronze pieces, generally dated to the two Anglo-Sikh Wars in the previous decade, where the natural wood of the carriage has been stained with varnish to a dark brown finish. It is likely that both styles could still be seen in 1857.