This bird is probably 100% leucistic. Eye colour is dark, and this is often seen in leucistic individuals of species with naturally red or yellow irises. The greyish plumage colour (instead of bright white) in this bird is in my opinion the result of staining (dirt) and not pigment. - Hein van Grouw
Grouw, Hein van (2006) Not every white bird is an albino: sense and nonsense about colour aberrations in birds. Dutch Birding 28: 79–89.
Grouw, Hein van (2013) What colour is that bird? The causes and recognition of common colour aberrations in birds. British Birds 106: 17-29.