I would say thats a diff breed of wagtail probably a yellow wagtail or even a grey there are many forms of these birds.

They start getting there new colours bit by bit each month im not excacly shur how long it would take but I would say by next spring it would have most of its adult feathers.

Here are some pics of a yellow wagtail in diff age's the one you seen may be a female.

Look at the right last pic thats a Rare gray wagtail if your lucky it may be one of them.