There is an article in the April 1989 Classic Bike on 'Buying to Ride, the Douglas Dragonfly' by Peter Watson. This gives the car paint equivalent colours for the stone and green as Stone; BMC El Paso Beige. Green; Vauxhall Conifer. Silver; Vauxhall Sebring. I've never tried to get hold of these but I suppose they will still be available, or could be made up.