There are not a lot of factory drawings of the tank transfers, but there is one for the E/F30 (1930 600cc), part #11383 and11384. It states - working inward to the panel field - a 3/32 inch wide black line, a 1/2 inch gap, a 3/32 gold line, and a abutting 1/8 inch blue line.
And the "panel field" on the 1930 S6 was... BLACK?
I spent last night reading the Motor Cycle from 1930.
The new Douglas program for 1930 was announced in the 24 October 1929 issue. The L3 (Lightweight 350) is discussed, but its finished is not mentioned, although by show time in late October the L3 was the only side valve Douglas with a painted tank. The "De Luxe and Overseas" 350 (the H3) had chromium plated tank with "black and gold panels". Both the S6 and the L3 were illustrated, both with Douglas blue tanks panels (if my interpretation of the b&w half-tone photos is correct!). The S6 at this time was shown with square knee pads (dating from the early 1920s), and there is a pinstripe well away from the panel on the chrome. Pretty 1929, actually.
By the time of the show things had changed. Presumably someone at Douglas liked the look of the black panel on the H3, and instead of using it on the H3 it was used on the big bikes: the S5, S6 and T6. The artwork for the Show Edition and Douglas advertising was changed: while the artist clearly started with the same photo for the S6, the airbrushing on the guards was different, and there were some small changes to the rear tail light. On the tank, the knee pads were gone, the tank panel was now dark (black?) and the pinstripe on the chrome was much closer in. The artwork is the same as that posted by Jonathan a few posts up, from the 1930 catalogue.
There is a full page Douglas advert in the Motor Cycle 5 December 1930, showing the four side-valve models in a grid: L3, H3, T6, S6/S5. Although b&w, the 350s clearly have blue panels, and the 600s black.
So my guess at the S6 tank finish is as per Doug's factory drawing, with a black panel: black stripe on the chrome, gap, gold line, blue line, black panel. Lovely.
I think an inky midnight blue/black would look great.
Surely there is an original paint survivor?
Cheers
Leon