I did some more research and indeed, the dark grey/dark brown camo was obviously introduced in 1937 until 1940.
Before, the Reichswehr and later the Wehrmacht used a 3-tone green/brown/yellow camo ( like in my picture) which was replaced by the grey /brown for new equipment in 37 and only became compulsory for all already fielded equipment by 1. July 39. This left only 2 months until the war against Poland, consequently not everything was repainted. During the war and afterwards troops found out that the camo was rather useless, the two colors being too close and consequently in June 40 grey only became the new official color.
So far so good. There are enough pictures proving a two color camo on vehicles of all kind, but only very little for tanks. Reason may be dust and dirt, but I still have my doubts, because 1) parade tanks look really monochrome to me and 2) why the heck was that discovered only 10+ years ago whereas only 20/30 years after the war people believed tanks to be grey only? Back then there were enough ex- tankers who would have known, one might think.
Anyway, an interesting topic, and I learned something, again!

This is the link to a German site, providing the official reference documents:
http://www.kfzderwehrmacht.de/Hauptseit ... trich.html