The correct colour for SAR Wessex should be RAF Blue-grey BS 381C-633. EDSG was the basic colour for FAA fixed wing aircraft but helicopters such as the Wessex, Wasp and Sea King were to be painted in the blue-grey. There is a very good article on FAA colours by Ian Huntley in Scale Aircraft Modelling Vol 7 issue 4.
If you are building in 1/72 then it is worth getting hold of the Modeldecal sheet for the FAA Wessex (Number 99) which is still available from Hannants - it confirms the colour as blue grey. There is lots of good information on the colours in them. Note that the FAA did also use Wessex HU.5s for SAR work when they had been retired from the commando assault role and these operated in their original olive green colour before being repainted in RAF blue grey.
The red bands on nose were originally dayglo before switching to signal red (or even roundel red).