Pond Herons in January

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:
https://bangkokcitybirding.blogspot.com/2014/02/pond-herons.html

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.

Update: Now in Ayutthaya and seen two pond herons with quite extensive summer plumage showing. Here's the most advanced (or is it retained?) with extensive maroon on the lower breast.

Further update: I'd initially considered this deep maroon colouring indicative of Chinese Pond Heron but Chinese is virtually black on the lower breast while Javan is deeper maroon in this area. I now think this is a Javan Pond Heron.

Javan Pond Heron Ardeola speciosa