Thanks for the quick replies. Resin, I had never heard the term "leucistic" before, thanks for teaching me a new word. Now I am wondering - for several months I have been noticing a very pale Canada Goose that I assumed was some kind of cross, but now I think it could be an example of leucism.
Yep, I've seen leucistic Canada Geese too. Leucism is birds (or other animals) with 'washed-out' colours, as opposed to albinism (pure white) or partial albinism (patches of pure white); there is also melanism (black), erythrism (redder/oranger than usual), and one or two others.
As a general trend, birds with colours deviating from the normal tend to be more frequent in urban areas, probably largely because predators (which tend to weed out conspicuous odd-looking individuals) are less common, possibly also because of mutations caused by pollution (not proven).