I found a link to a white wagtail with a yellow face this is what it said:
"Adult male Citrine Wagtails are quite gorgeous birds with dazzling yellow heads bordered by a neat black scarf. The yellow extends down the rest of the underparts, the upperparts are basically grey with white wing bars.
In other plumages they show variable amounts of yellow but the impression is usually of a White Wagtail with a yellow face. To confirm that your bird isn't a Yellow Wagtail, look at how there is a complete pale border around the ear coverts. Juveniles in autumn can be completely devoid of yellow and so look even more like White Wagtails, but the lack of a broad, dark ring across the breast should prevent confusion"
I wonder if yours is one of these
Here is the link
Motacilla citreola