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