Now, for a really prototypical look for the guns, they ought to be in covers, either the "mustard" tan Army covers, or a grey "oilskin" canvas. What color grey? Pick one, some of the covers were made up by salimakers or Bosun's Mates from the on-board stock material.
M2's will be a grayish parkerized color, with barrels a least bit darker--like a quick rub with a 2B pencil, maybe over a half-steel, half-panzergrau color. Grips would be black or a bit like Testor's Rubber. The mount down to the actual pintle would either painted like the nount, or a brassy-sort of bronze. Ammo boxes would be either that odd QMC green, or a Haze or Ocean or "battleship" grey. M1918's will be darker in the reciever, and grayer on the barrel shroud. The mount & cradle to the pintle will typically be a contrasty "gun" color, or a bronze sort of color.
Props, as already stated, will be dark; you could go with a dark bronze or a dark gunmetal color. A bit of Oily Black over Steel is good for the shafts.
The wheel is steel and painted or covered in half-hitched cord with a turk's head at TDC. A steel wheel would likely show darker, but a tad glossier, by hand contact. Wrapped in cord, it could be from tan to grey to a whitish color.
Life rings were all sorts of colors, from period references; but, to me, a near-white or pale grey "reads" well with generally camoflaged LC (remember the issue kapok life jacket is a dark blue-gray very close to panzergrau when new). Fire extinguishers are brass with bronze & copper highlights, too.
Now, a painted-to-match ship camoflage LC can look a bit odd by itself. You can "dodge" that the least bit by over-painting over a slightly darker base color to "show" the wetted are of the hull. Or, painting the hull bottom in the darkest camo color, and carrying lighter colors oer that does not look too bad--if the least bit aprototypical.
LC get a lot of use and abuse. And they were out of the water a lot. And idle hands get to mischief; so they would be painted. Often. Now, the one thing I've never quite managed to do myself is to get the look of a brand-newly painted repair right. Just something about a 4x8 patch of brand new paint that always says "working landing craft" to me.