I managed to find a list of shipping lines colours from 1927, thanks to the Compact Encyclopedia. I've attached a thumbnail scan of the image here. Naturally, the image is *much* bigger, but I've had to place a small version here because of image size constraints. If you want a full sized version, please let me know.
At the moment, I'm fiddling with the image parameters, trying to find a compromise between size and quality (the best scan I have is about 200k, but the smallest ones result in the annotation being slightly illegible).