Unusual colourations "sports" can happen in most wild birds some more than others, chaffinches are one of these species, others I've seen were blackbirds, jackdaws, hooded crows, a white moorhen and many others
Always interesting to see
This can also happen breeding birds in captivity and when it does we can capatalise on it and start new colours, remember all budgerigars in the wild are green, look at the colours now
In the wild however they usually live a seaon or two then disappear