In Chapter 2, the truck is colored red. Red will be important for a number of reasons throughout the novel. Red can be interpreted both literally and symbolically.
Literally, red is the color of the earth in Oklahoma (Note the use of the song "Red River Valley" repeatedly in Ford's film.)
Symbolically, red represents the blood of the people who have lived on the land for generations and the toil that they have invested in making the land fruitful.
The fact that the truck too is red probably has to do with the blood of the people being sold, as it were, for profit. The truck is a transport truck, brand spaking new, and the Oklahoma Transport Company is thriving because of the changing situation of the migrants and their necessity of spending money on the road.