Shades of Grey

Now, I like to think of myself as enthusiastic and reasonably competent birder but I am by no means an expert. I struggle with identification at times - especially with some warblers and gulls. In fact, at times I have made a complete arse of myself.  I spent an hour once tracking a skulking bird through the scrub - convinced it was a Dartford Warbler, only to discover I had been wasting my energies following a ruffled and somewhat irritated Dunnock. I don't know who was more disappointed - him or me.
So, forgive me for what is about to come.
Yesterday at the Orchard Centre I lazily snapped off some photos of 3 Pied Wagtails perched on one of the fences. When I looked through the photos later it was clear that there was a marked difference in colour between the birds.

One was a particularly handsome defined black and white individual while the other two were quite grey and indistinct by comparison. Females? Young birds?
Then I got thinking..... speculating.....dreaming....fantasising...getting carried away (Dartford Warbler moment coming up). Could they be the alba (White Wagtail) birds that are often reported.
I've checked the books, trawled the Web. I'm still clueless. I'm really struggling to see the differences between female pied and white wagtails.
Best guess is the grey birds are female Pieds, but please correct me if I'm wrong.
All answers on a postcard to the Lone Birder, Hartpury Orchard Centre, Glos...........