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.
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.
|Chinese Pond Heron Ardeola bacchus|