This video from Britain says about itself:
BTO Bird ID – Yellow-coloured wagtails
17 June 2016
When is a yellow wagtail not a Yellow Wagtail? These bright-coloured summer visitors are declining across much of their range and a frequent mistake is believing that any wagtail showing yellow in its plumage is this species. This video will help you separate individuals from the more widespread resident Grey Wagtail – which despite the name always shows yellow! – and even juvenile Pied Wagtails.