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.

This is Motacilla flava flava of western and central Europe which has a blue-grey head with a white supercilium (line above each eye).

1st & 2nd photo: Male & Female. Zemplén Hills, Hungary, 28 April 2019
3rd & 4th photo: Hortobágy National Park, Hungary, 1 May 2019
5th photo: Borsod Mezoség Nature Conservation Area ("Little Hortobágy"), Hungary, 1 May 2019