[Vekke]

Yes if the coloring is different than the blue on top of the truck your car will face that 6 m/s slower air and use less fuel. Now It would be also cool to know for how long/ far behind that green 6m/s slower air will go. I have estimated the effect with that 3 second distance to 1-6% savings. If you drive 5m/s slower 18 km/h or 11.1 mph slower 55 MPH -->50MPH your fuel consumtion is about 13% less according aero rolling resistance calculator.

From myhtbusters test they found out the following results in their tests: Wer can now put those distances to that simulation and get following colors
100 ft=30m=11% greenlight and green dark so about 17.5 m/s air speed--> 7.5 m/s saved 11%-->1,46%/(m/s)
50ft=15m=20% yellow air mostly 13.4 m/s
20ft=6m=27% orange air mostly 7,8 m/s
10ft=3m=39% red colored air mostly so 5m/s

If you calculate the difference mean that 6 m/s would mean about 8,8% fuel saved at that distance. For better perpective you need also the upper picture because the truck is narrower and the green is not so long in width section.