The fuel tank is steel; I asked the same question back in January when I visited the Yamaha facility here in the NW Atlanta area to see the R3 and R1M. Theye had a pre-production R3 in blue--gas in the tank and a license plate on back and the tires were scrubbed. I didn't get offered a ride, but it had been ridden on the street recently.
The sides are covered with plastic body plates. I plan to paint the black center portion on my hopefully-soon-to-be arriving red model in matching white. The blue has silver in the center.