Khaki Green No. 3 was in use 1939-1941 by Britain, but the Canadians had stock they used into 1942. As Robert says, the switch from green was due to the need to reserve Chromium Oxide for aircraft paints. From 1941 SCC2 was the base colour until April 1944 when SCCC 15 was ordered.
It is also worth noting that British crews did not paint their own vehicles. Re-painting was done by REME when a vehicle was in for repairs/refit or by specialist maintenance units. This is why some vehicles were still in older colours after the introduction of the new ones.
The Centaur is a good example of this. They were intended to stay on the landing craft and only support the beachhead from there. Therefore a repaint to SCC15 was not considered necessary. In practice they were landed and sent forward to support troops, in their SCC2 colours.
Interestingly, look at the hatch on the front Glacis in these pics. The inside is white on one, Scc2 on the other… never say everything is done by the book…
Now compare those wartime pics with this ‘restored’ vehicle painted ala Tamiya’s model’s paint guide ‘XF-52 Dark Earth’:
What type of chocolate do you prefer - Milk or Dark???