[Bodston]

From my copy of 'Blitzkreig - Armor Camouflage & Markings, 1939-1940' which arrived today. Steve Zaloga says

The exclusive use of dark grey as an economy move for the duration of the war was officially mandated in an army directive, HM 1940 No.864 on 31 July 1940, but it is clear from photograpic evidence that this practice had been in effect for some time.
The grey adopted by the Wehrmacht was a very dark grey with a slightly blueish tint. In sunlight, it became a chalkier, lighter shade. The grey was so dark, in fact, that Pz.Kpfw. I's serving in the Condor Legion in Spain were popularly called 'negrillos' ('the black ones'). The adoption of this colour is perplexing. Dark grey was ostensibly chosen to make the vehicle blend better in the shadow of buildings or trees, where it was supposed to be parked when not mobile. Yet a dark brown or green are equally effective in such settings and are more natural in other surroundings whether forested or open. It is speculative, but entirely possible, that the grey was chosen as a subtle means of national identification, similar to the attempt by most nations to adopt a standard infantry uniform colour and design that would be distinctive and unmistakable. As Germany's likely foes (France, Britain, Poland and Belgium) relied on various shades of dark green as their predominant vehicle colour, grey would serve this function. The Wehrmacht's lack of interest in vehicle camouflage at this stage of the war reflected the offensive tenor of German tactics.

Seems to sum it up nicely.