I did some more digging. According to Tomas Chory’s “Camouflage Colours”, the dark grey and dark brown (Dunkelbraun) camo was ordered pursuant to H.M.1937 No.340 on 12 July 1937. However, he says that by the invasion of Poland most vehicles were in plain dark grey - mostly to save time and money. So by the invasion of France, few were in the 2-colour scheme. The plain dark grey was formalised in July 1940. This conflicts with authors like de Sisto, who UVS mentioned.