April 1944 saw the issue of the ACI introducing SCC 15 Olive Drab this was reflected in the RAF by an AMO in june 1944 but it is likely the first vehicles to be factory finished in SCC15 would be rolling out of the factories at the very beginning of 1944, and the same was probaly so for specialist vehicles such as cranes & bowsers having been to an MU for a major overhaul.
SCC 15 was very green see the 2 video captures attached. The crane is SCC15 Olive Drab and SCC14 "black" disruptive. you can see the green tinge in comparison to the green on the Lancaster. Note also the yellow upper surface on the crane jib. This is from the video filmed late 1943 early 1944 at Hemswell by the Base Commander. At the time the runway caravan at Hemswell was towed by a Morris Light Recce car that had its gun position removed, the caravan was the mandatory b & W checker but the tow vehicle was overall golden yellow !!
I have virtually every Humbrol colour but for the last couple of years I haven't used Humbrol to do a complete military vehicle, For WW2 civvy fire service models ( gloss dark Ad Grey) I use Humbrol. For my post 1949 fire trucks I used Xtracolor reds. and as I said for my military I have either mixed the colour following Starmers info or bought White Ensign paint. I also invested in a Badger paint mixer its a little battery powered mixer absolutely great cost about £15 with P & P . A great one off investment runs on 2 AA batteries-it works better at slowish speed so I use flatbatteries from my wifes walkman type machine. Because my models normally contain resin white metal plastic and even wood I always prime everything using Halfords own brand grey primer in an aerosol. It makes a real difference.