It's possible the RLM 27 ( FS 33591 ) is said because it was a colour of a quite close tone to the Italian yellow Giallo Crome 7 ( FS 33655 ) that was used for ID markings. The RLM 04 is said to be the FS 33538 and was of the orange tone contrary to the RLM 27 that was of the lemon tinge. However the German paints were used as well and you might be right.