Hi,
The Vintage Carriage Trust has the only 3 surviving Metropolitan Dreadnought coaches in it's collection. The trust is based at Ingrow on the Keighley & Worth Valley Railway.
http://www.vintagecarriagestrust.org/Collection.htm
Their website states that the Dreadnought coaches were originally teak & then painted in various liveries over the years - the final colour being London Transport's dark brown livery. Two of the VCT's Dreadnoughts are currently in this livery (Nos.465 & 427), whilst No.509 is running with the teak finish as built. The website states that No.465 is on an extended loan to the Buckinghamshire Railway Centre and Nos.427 & 509 are at Ingrow.
This doesn't really give details of the dates the liveries changed, but it could be worth making contact with the VCT for more information.
Tony