Good stuff.
Note that the lighter coloured 'stripe' on the bottom of each roller door, was intended to show if there was any 'creep' when the doors were closed. This 'creep' was the possibility / probability, of the doors not being closed properly after the bomb run, which could cause aerodynamic and fuel economy problems, with the 'stripes' being designed to be observed by other aircraft in the formation, and, on return to base, by the ground crew.
Due to the possible, and very common, build-up of fuel vapour in the bomb bays, once on the ground, the bomb bay shutter-type doors were normally cranked partially, or sometimes fully open, to allow fumes to disperse, as the fumes could very easily ignite due to static electricity, or a small spark, which could easily be generated by the 'making or breaking' of any simple switch in the aircraft.
When the doors were cycled to close, a crewman, very often the top turret gunner / engineer, had to physically check that there was no 'creep', where the doors might have either not closed fully, or perhaps had moved back up slightly due to slipstream pressure and, if there was movement, he had to then manually crank them into the desired position.
These stripes were normally white, or yellow, with yellow being more common on NMF aircraft, due to it being more visible against the bare metal, although both colours were used on painted aircraft.