My old man painted his logging pole trailer, truck bunk and headache rack white a long time ago. Looked sharp as hell but after a winter hauling season it looked like crap as the paint wore off.
Honestly with those colours I think gray like the truck would look best. Truck's colours are too neutral for any colour IMO.
Paint never stays on frames or trailers in the bush. No matter how much prep work goes into it the paint never looks good after a 10 month haul season lol.
I would go black, easy to keep touched up with common spray paint. Gravel is hell on the rear frame/axles. At the very least I would go with a color that can be matched in a spray can. Blue frames look great, when new anyway.
Wife said blue, cause I was gonna do blue LED ground effect lights under the truck. Like her idea, but there could be repercussions if any other idea is used. Just sayin.