Is it fanciful to think that Javan and Chinese Pond Herons (Ardeola speciosa/bacchus) might be separable in the depths of 'winter'. Both are in non-breeding plumage and by all accounts are inseparable. But...
A bit of background:
Here are some photos from Lumphini Park, Bangkok.
The difference in appearance of the last two is striking and the colour balance of the first two match what was seen in the field.
At the moment this is just kicking around the idea but as the linked article above suggests perhaps it is worth giving some more thought.