Originally, radomes were black because it was a special radome paint that needed less preparation and primer and had less attenuation effect on the ground mapping radar returns. Later developments in paint technology included coloured pigments. As to 617 having a unique paint scheme - I find that hard to explain as, during most of the time the Vulcan fleet were wearing camo on top with white undersides, the aircraft were allocated to the "Wing", were centrally maintained and were shared by all squadrons. In fact they swapped stations pretty often too.
As to matt finishes, although many Vulcans appear to have a matt paint finish in photographs, they actually came out of the paint shop in gloss, but it weathered very quickly to a matt appearance, a result of sixties paint technology - and an effect often seen on sixties motor cars, especially Vauxhalls! You would see much the same thing if you looked at a lot of photgraphs of VC10s during the sixties and early seventies at the time 10 Squadron were referred to disparagingly by the Britannia squadrons as "Shiny Ten". Shiny fresh out of a Major and already matt before the next Minor.