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Due to it's low-level strike role, when retired from the 'Nuke' role. It flew following the contours, and would roll through valleys, for example. All low-level strike aircraft in the RAF had similar schemes - Harrier, Tornado, Jaguar, Bucanneer etc.
I sort of understand that theory but I used to see and hear them doing low level training on the North Yorkshire Moors. Phantoms Harriers and Provosts used to make a whistle and then a boom and roar as they passed the Vulcan just made a deafening roar at all times. I think they should have painted "Up Yours Ivan" underneath, there is no way camo would have helped at all.