50th Anniversary Duel truck.

[Force]

Union Oil appearently ordered some trucks in sand tan but most of them were dark blue at the time, Union Oil also had some trucks operating (directly or through independent contractors) in such a color.
It is possible that the Duel tractor was repainted light tan to operate in that fleet later on, or upon sale, as a base for the gold, and it was treated with lots of dirt, oil and grime as it was meant to look mean and evil...the look of the truck and the red "flammable" lettering on white on the tanker trailer was described in the screenplay wich was written in August 1971 before the movie was shot, the movie was shot under 12-13 days in mid to late September 1971.
The main 50's (some say 1955 and some say 1957 depending on source) 281 truck used for most of the scenes in Duel was destroyed in the final crash that ends the movie so it's gone, a backup 1960 281 truck with trailer was built as the main truck was acting up sometimes, but it was appearently never used in the movie...if so very breefly...remember the movie was shot on location in only 12-13 days according to the interview with Steven Spielberg in the Blu-Ray copy I have of the movie, so there wasn't much time.
The 1960 backup truck is the surviving Duel truck we have with us today presently owned by Brad Wike and it is not exactly like the original main truck was, and I have read that it was repainted and used in other productions, and then done up like the Duel truck again for something else before it was sold off.
A third truck was made up to shoot the additional scenes for the longer theatrical version of the movie in 1972-73, this truck was a 1964 351 and it is also slightly different from the original main truck, the scenes where the third truck was used was the exstended gas station/laundromat scene, the schoolbus push scene and the railroad crossing push scene, this truck was later destroyed in another production and is also gone,  the Valiant in these scenes was a year newer as the original car was destroyed in the final crash.
The tanker trailer used in the movie is most likely not the same as the truck originally had when it was operated by Union Oil as it's an older piece from late 40's - early 50's.

Edited April 19, 2021 by Force