a bit further down -
I'd say that a very light gray was the usual colour, later compartments had different colours for faster orientation.
"Alarm !" is perfectly right, e.g. when an outlook saw a plane. Then as soon as the alarm bells rang the whole crew would run to battle stations and prepare all for dive, and "Fluten !" would just be an order given in the central control room to the men at station there, meaning flooding the tanks by opening the upper tank's valves with the "christmas tree", so the air would be blown out by the incoming water from underneath.
Certainly, all actions has to be fine tuned by the whole crew like running to the bow, opening forward valves earlier so the the bow would cut under first and the propellors drive the boat down dynamically, helping in a fast emergency dive.
As well in an emergency situation the men in the engine rooms would already close the exhaust valves for the Diesel engines without waiting for the flooding command, often giving a last full speed ahead before shutting down.
With the buoyancy tanks already flooded for a fast dive and a well-trained crew, a type VII boat would be able to completely submerge in less than 30 seconds, even a bit faster than modern boats - only that modern boats are not supposed to spend much of their time at the surface ..