I understand all this, and know about the Louis' portaits. I just don't figure how could that be in Russia in spite of its tendency then to Europe-ish behaviour...Need to seacrh for some more example of such Russian portraits.
As portraits of the royal family from Peter the Great onwards clearly demonstrate, western dress and standard western royal allegorical allusions were the norm. In the portrayals of Ioann VI Antonovich as Emperor he is in a wide variety of clothing, some of which mimic the coronation ritual - plain linen with royal robes, which was very usual in western royal iconography; there is another royal portrait type of a standard western nature where he is in classical roman soldier dress; and the informal portrait with his nurse (?), governess (?) where he is in the western dress for little boys which suggests that this is what he wore normally. The portraits of his parents are in the normal style of western court dress. The single portrait which stands out is that where he is dressed in a short red smock, with a cat (?) by his side, with no marks of his imperial rank, which looks genuinely russian rather than western, but as I said that seems the exception rather than the norm. Tsarina Elizabeth, who ousted him, made a great feature of her russian heritage, was never depicted in royal portraits in anything other than western dress, and I would have said that the main aim of the royal portraits of Anna Leopoldovna's short regency appears to have been to continue what had become by then the normal type of royal portraiture for her son - which was almost overwhelmingly western. Was there a particular reason why you think Ioann VI Antonovich would have been dressed or portrayed in anything other than the western style which by then was normal for the imperial family?