[MJN]

My guess is they used a preproduction model in their advertising. There is no mention of painting that area under the grille any differently in the Vehicle Info Packet (page 16 of the pdf). For solid color paint all areas of the cab, front end sheetmetal and box are painted the same color. It also gives detail on the conventional and special two-tone colors and application.
https://www.gmheritagecenter.com/doc...olet-Truck.pdf

On page 13 of the pdf file there is mention of the silver painted headlight bezel centers.