Depends on the period, as Dn Jackson says for the ACW period pained the trail colour.
I've adopted this policy for guns from 1740 onward.
Prior to that, wood.
Sorry Broglie, they were never made out of metal.
The main euipment used on a gun firstly, the sponge/rammer – one end, the rammer, is plain wood, the other is the sponde covered in a fleece. Dipped in water they are used to mop out the barrel after each shot to extinguish any embers left from the last shot and prevent a premature discharge of the round. After a few shots this fleese is blacked, dark grey drybrushed black would be the correct color. The man who uses this equipemnt stands in front of the righthand wheel. It mans when he turns and ram the round home he can use his right arm to push the round home. Unlike the movies you only use one hand for this job. The other tool used by this crewman is the worm, a rod with a double helix screw, used for extracting shot that will not fire properly. This is a dangerous proceedure. I was on a crew once when I was a gun crew, the problem was we had a explosive round in the barrel. One spark could have set of the fuse since it's basic compound is black-powder.
The man in front of the left wheel is responsible for adding the shot to the barrel.
The trian spike, is used to make minor adjustments in the direction the gun is pointed. The gunner who aims the piece and the designated crewman work together on this. Depending on the period either the man from the rear right of the gun (ACW) or with some Napoleonic drills the left of the gun.
The last member of the four crewm main crew fires the gun. For the Napoleonic preiod this uses a linstock. A short rob with a metal head. Rapped aount this linstock is a piece of slow match. The slow match basically rope soaked in saltpeter so when lit one end smoulders and is used to set off the gun when primed. The slowmatch should be painted a creamywhite color. The linstock a darker wood colour.
There are two other crew members around the gun, one is the runner who brings up an individual round and the gunner, an NCO who actually aims the gun and gives the fire order.