Hi Pierre, old fellow!
For your information, the IV./JG1, then I./JG1 "Teufel in den Wolken" emblem was already depicted with "yellowish" background instead of blue in serie "Jagdfliegerverbände der deutschen Luftwaffe" (Vol.7 + Vol.10/II) already from 2008 on.
But, that said... the devil always hides in details!
It is an old emblem inherited from 2./JGr.101, where it was sported on both sides of Emils during the Sitzkrieg (crest background is also depicted as yellow in Vol.2).
For IV./JG1, on both sides for Emils, and on left side only for FW's (Vol.7).
For I./JG1, on both sides for FW's until mid-43 (Vol.10/II).
A small detail : in most of pics checked, the devil's elbows always cross the emblem's crest outline for JGr.101 as well as for JG1 Gruppen.
In Vol.7 (p.124), you clearly can see this emblem on the White 12 (W.Nr.437 from 10./JG1) in Deelen - summer 42 - with a very dark background, but it is the only b/w pic I ever saw with such a dark background. Then maybe Mombeek got an original color pic of this aircraft. So it sounds like his statement could be verified, this is the assumption I took so far.