Correct, UP repainted the trucks to Silver later. Also the nose antiglare changed colour from Grey to Green after a while. That is one of the visual differences between the AZL and Märklin version.
The trucks are easy: the change from grey to aluminum was ordered in 1955.
The nose antiglare green is a bit more nebulous. There was a 1951 order to paint all hoods the non-slip green, which probably took a year or more to execute. The locomotives that were delivered in the ‘46-‘49 time frame were green replacing yellow, but there doesn’t seem be clear data between those dates and the 1951 order suggesting there were locomotives without the green.
I've been poking around in some UP images and while it can be challenging to differentiate between green and grey, here's what I saw. Images from '53/'54, various locomotives, with grey hoods with the same footprint as the green. Interestingly all trains were on an LA route. Found grey hoods in the early 1960s with a different grey hood, one that includes the sill of the windshield.
So a descent amount of evidence that the AZL representation is reasonable.
It's interesting that the trains sporting a grey hood, COLA, Challenger and mail train were all LA trains. Perhaps they could be for a future COLA ?
And if AZL is willing to give up its secrets, I'd like to know of a couple of prototype images used for the model.
And a useless bit of trivia: most of the PA-1s shared their road numbers with a group of 2-8-0 steam engines until the last steamer was retired in 1956. The 907 model was one of them.
The Ranks and Kratville Streamliners book says UP started regearing the PAs for freight service in 1955, but they are not clear how long that took. Your picture suggests it was not immediate, although they state that PAs were not used for passenger service on the Nebraska line after 1955.