I can't access my WB30 data at the moment due to a computer failure and upgrade however-
The time span of the WB30 may be quite long, forgetting the prototypes there is the initial trial batch of 50 which I seem to remember are listed in the BSA factory ledgers which appear to have been returned to BSA for a serious upgrade which meant amongst other upgrades new engines being fitted and possibly a repaint?, the dates of these appear to be 1940?
Then there are the GPO and Royal Navy Contracts, the dates mentioned in O&M are 1942.
So its quite possible KG3 was used on the early batch and SCC2 Brown on the later batch, this original footrest may just hint that the early ones were green?
But in my Royal Enfield Register I have a couple of motorcycles that can give us some more precise information about the date when Enfield started using S.C.C.2 (because their stocks of K.G.3 had run out?): frame number # 12828 was despatched on December 1st 1942, and was still painted in khaki. Frame number # 15432 was despatched March 27th 1943, and was originally brown...
The fifty, according to the factory records were mostly delivered during April 1941 (a few in March and May). This is certainly KG No.3 period.
The Ministry of Supply records show the GPO contract as dating from February 1942 but with no indication of delivery date. The Army Council Instruction indicating that SCC No.2 Brown was to be the new basis colour was dated 30th May 1942. In general, these instructions seem to have confirmed new practices rather than announce them, but of course existing supplies were to be used first so dates could vary between manufacturers. "Mid-1942" is probably as close as it's possible to get.
The RN contract was C (or S) 13872. The subsequent contract S13873 was for 35 RN M20 sidecar outfits (WM20 71449 - 71483). It would seem resonable to assume that these were delivered prior to C13290 (WM20 71818 onwards delivered late 1942, into 1943)and which should have been the first WB30 contract (WB30 1001 - WB30 6000).
I think there is a good chance that these mid to late 1942 WB30s were indeed brown.