From the Disney Parks Blog…
Walt Disney Imagineering costume designers worked in collaboration with Chevrolet to design costumes that reflect style and technical innovation. Elements of the automobile design experience in the attraction are found in the colors of the costume, while the silver/gray base color was chosen to represent one of the many colors of Chevrolet automobiles. The blue and black accents found in the Test Track graphic reflect the field testing required of all new vehicles and the color changes that take place inside the car while experiencing the attraction.
The new Test Track opens on December 6th but Cast Members in the new costumes can be found at Epcot now outside the attraction to greet guests and answer questions.