I have posted a scan of two sheets of a BP Shield Magazine from 1968 which explains why they changed the colours. If I posted them here, chances are the font would be too small. If anyone still cannot read them, and is interested, I can translate in this thread.
http://www.shipsnostalgia.com/gallery/showphoto.php?photo=9866
http://www.shipsnostalgia.com/gallery/showphoto.php?photo=9866