This species' systematics and genetics are extremely confusing.
Literally dozens of subspecies have been described at one time or another, and some 15-20 are currently considered valid depending on which author reviews them.
These subspecies vary mainly in colour and head pattern of breeding males, whereas females and immatures are difficult to identify.

The first 3 photos show the male of the South East Europe subspecies of the Yellow Wagtail named Black-headed Wagtail (Motacilla flava feldegg) for its distinguished black cap.
The 4th photo is very likely a female Black-headed Wagtail.
The Motacilla flava flava of western Europe has a blue-grey head with a white supercilium (line above each eye).
The British subspecies, Montacilla flava flavissima, has a yellow head (the male) with a brighter yellow supercilium.

1st, 2nd & 3rd photo: A male. Lesvos, Greece, 1 May 2010
4th photo: A female. Lake Koroneia, Thessaloniki, Greece, 31 July 2017