Ivan, I think it is unlikely that the GSP was originally flagged with white only. I have previously corresponded with people from New Zealand (where white flags are used) and they reported that they have never flagged a GSP (which I think is rare there).
Your GSP may have had a very faded yellow flag (NW Australia), but probably the most likely is that this is a Chongming bird (black and white) which has lost the black flag. This was the final conclusion of the bird I saw last year, which I saw on 27th March and may even have been the same bird!