@craigindaytona there are as you say different colours, but as far as I know it is not paint, it is the colour of the material used to make the nosecone, as they need to be easy for the radar to penetrate. Same goes for all fighters with a radar. Sometimes, often late in their career they could get painted in some special colours, like the last two RAF Phantoms to patrol the east-west german border – they were painted all blue. In 1972 the Royal Navy FG.1’s made a farewell tour in the states with the radomes partly painted (and 001 got “tagged with ROYAL NAVY replaced by COLONIAL NAVY, and the brit roundel got changed, the white part extended to make a star, resembling the early USAAF markings! – sorry straddled away a bit – so back on track; another exception was FGR.2 XT853 that had the radome replaced with one in unpainted steel for some trials in connection with the Concorde, robbing them the use of their radar. A quick search in my library did not reveal the text, but it is obvious that the ring – as described by Robert – is for a static port. So in conclusion, yes the radomes were left unpainted Black, Grey, White or Tan in operational service with the exceptions of special use and special colors. The color used more or less depended on the camo – like the Falklands grey ones having tan/light grey radomes. Do also note that radomes (most notably the black ones) faded in use, and repairs are very visible.
(On another note, Xtradecal has just issued a set of markings for the “COLONIAL NAVY Phantom today! – just saw it on hannants under new arrivals)
Hope this helped – and it is all open to further comments as I do in no way claim to be an expert on the matter, so this is just my pennies worth.