Its the area where the front fenders were. It looks like it was green, see the front of the cab.
The red paint line follows the inner fender the other or outer side is green.
The factory didn't paint the cab with the fenders or inner fenders on, way too much masking needed.
With the front clip on the guy with the paint stick could not be writing on the fire wall so easy either.
There used to be pictures on line showing the painted cabs with no fenders on them riding on an over head conveyor
with padded hooks holding it by the upper door frame.
Hope this link works. Probably have to copy and paste it, its the "Cab drop photo" it shows the front clip being installed after cab and engine are on the frame. Shows some nice shiny black frames too.
Link