Got to say none of these explanations are very plausible. Why would one choose a derogatory nickname for a rival service that hinged on admitting ones own grubby habits and poor hygiene?
As for "crab grey" I have been unable to find any reference to the colour being so described...Although I did discover the concept of painting warships in a low-vis scheme was pioneered by the Imperial German Navy -demonstrating again their unsportsmanlike attitude to warfare.