Landships II

Looking at the same Whippet image, the upper angled gas-tank panel has a darker color (although less-so than the black) on the far-right quarter, and something lighter in tone over the left 3/4.  However, the vine has a short stem angling down under the star, and it appears to be the same color on either side of it.  This is also true for the front panel of the engine housing.  Your proposed brown and green seem reasonable for what we can see, but that could be a lighter color ("khaki?") on the lower front plane of the fighting compartment, behind the bonnet support.  The border black seems almost snake-like here.

On another note, there's a flat-iron bar near the top of the driver's front roof panel that has puzzled me.  I skimmed through the Westerbeke monograph, but didn't see any mention of it; there's no coverage of the cab roof exterior.  I first noticed it on the American Whippet (image attached), and assumed it was damaged due to the angled position.  However, I've since noticed a similar angle on the South African, Belgian, and Japanese Whippets.  The one at Bovington is removed in most photos, and dangling due to a lost/broken bolt in one (attached).  However, it has an enclosed bump lower down on this roof plate that I don't believe I've seen on any other Whippet.  Photos I've seen of the AA kit are inconclusive, and the instructions don't show either piece.

Since it isn't round, it's not a grab or handle - such as on the engine covers.  I've thought the most likely use to be a footrest for some specialized task, but don't get the angled design - unless that's commonly occurring damage.