It's an interesting point which I've enquired about as well and I've never found any official reason for it. Best guess is that it was simply deleted when the later paint schemes were introduced. It's interesting that 617 Squadron adopted a black-less radome whilst their aircraft still wore the grey/green and white paint scheme but this paint scheme was virtually unique to that squadron, so unless one imagines that their radar equipment was different to that used by all the other squadrons, it must have simply been a change in taste rather than being down to any technical reason. It might be worth noting that the VC10 fleet lost their black radomes too, and again I haven't seen any reason to suggest that this was down to anything other than aesthetics - or perhaps a desire to save a bit of money!