Purple Heron

First of all, all Egrets in India except one are either grey or white only. The exception is the Cattle Egret which is white but when breeding gets a golden head, neck and back, but it has a short bill.. Secondly, some Egrets and Herons are very similar and the Ardea Genus of herons includes the Great White Egret. Your bird has a very dark head, almost black, with a dark line running down the nape. The back is dark and light brown with a touch of grey so beginning the change from juvenile to immature. The only deviation from most Purple Herons is the dark upper mandible which usually shows some yellow at the sides, but this could be because they start black and change as they grow.

I'm guessing juvenile Purple Heron.