Pied Wagtails often have a variation in colour, but they can be different types
(alba) showing more white and (
yarrellii) the more darker. Suggest you look it up in the Collins Bird Guide for a better explanation.
Click on photo to enlarge.
Whiter one in my garden last week
Darker one at the Naze today
A second different one, whiter with a hint of yellow, in my the garden
Darker one at the Naze again today