I attempted a bronze color for my canons and a dark cast iron look for my mortars. I didn't exactly succeed on the bronze, but the color I ended up with looks better with my army than the slightly green pale gold look of real fresh bronze that I was hoping to achieve.
I don't know if all these colors are available anymore from GW, but I also used some non-GW colors.
The cannon got a base coat of Scorched Brown, then a coat of Brazen Brass, then a highlight of Dwarven Bronze, then a 5 to 1 mix highlight of Dwarven Bronze and Boltgun Metal. Then a 5 to 2 mix highlight of Dwarven Bronze and Boltgun Metal to all the really highpoints of the details. For the inside of the barrel and some dry brushing on the end of the muzzel, I used Reaper Master Series Scorched Metal. After the model was fully painted, it got the dip. In my case it was Minwax Polyshades Antique Walnut polyurethane stain. This is a really rich brown color. If you don't dip you could use a wash of Chestnut Ink mixed with Brown Ink, ratio of 5 chestnut to 1 or 2 Brown Ink.
For the mortar, I used a Chaos Black base. Then a 50/50 mix of Chaos Black and Boltgun Metal over everything. Then I used progressively more Boltgun Metal to the mix as I highlighted it more. My final highlights were pretty close to being pure Boltgun Metal. As I couldn't find the Reaper Master Series Scored Metal while I was painting this one, I just painted the inside of the Mortar with a couple watered down coats of Chaos Black. Enough that it wasn't a solid black coat, but just hints of the metallic showed through. As with the rest of the army, it then got the Antique Walnut dip.
I also did a wood grain pattern on their chassis. I succeeded more on the great cannon than I did on the Mortar.