This topic has been moved to
TYPE VII. It's probably not the best home, but this is definitely Uboat related and deserves to be amongst uboats as opposed to General Modelling.
Does anyone have any info? It's an interesting question. My uninformed view is that they splashed on "some color" onto raw concrete just in spots...but I'm going by faded Wiki photos.
http://models.rokket.biz/index.php?topic=1472.0