Freeman listed the color as dark purple in one of his books.
Here are some earlier observations:
Posted to HyperScale, 8 November 2003>>>>>>>>>>>>>>>>
The whole "plum" P-47 story surfaced here on HyperScale on November 17, 1999. Gregg Cooper posted the following (retyped by me, since I only have a hard copy printout:
"This is second-hand info as related to me by a friend of a Wolfpack crew chief. (I know, I know, but it is a good story...)
"Anyway, The 56th was visited by some big-wigs in the US automobile industry, maybe Chrysler.
"When asked if there was anything that they could do for the Wolfpack pilots, the pilots responded with a request for some automotive paint that they were fond of. The color was a deep 'plumb', a dark reddish blue.
"The paint was delivered, and made its way onto some very intersesting airplanes.
"In B&W, the color photographs very dark.
"There is some color movie footage, and quite a lot of color film prints out there that will show this color. Again, though, any photography from this period is suspect of course. I will attempt to put more proof to this tale, and will let you know. Anyone else out there ever hear of this one?"
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So, that's all I know, could be true, could be rumor, could be an out-and-out lie. But it sure sounds cool, and I'm going to love building a purple P-47M if I ever get a chance to build again!
Cheers,
-Dana
<<<<<<<<<<<<<<<<<<< Following response from HyperScale:
Piotr Dmitruk plum color Sat Nov 8 14:51:11 2003 213.25.15.62 Lanowski himself said, that his aircraft was painted in plum color, just like his car was. It's possible also for Gladych's aircraft. But instructions for Techmod decals claim for using Humbrol 189 (Insignia Blue). Lower surfaces were in NMF with highly polished leading edges for adding speed, and as I can recall- without underwing bomb pylons. HTH Greetings Piotr
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The argument may never be settled for all modelers, but that leaves more possibilities when you make your own choice. (I mixed a batch of purple many decades ago, but never finished the kit...)
Cheers,
Dana
emoji, with the likes etc.! 
