It's always exciting to look out the window and see a new bird in the yard, but to see a new bird and then not be able to find it in any of my "Northeastern United States" bird books was even more exciting. And what a pretty bird it is. It's been here for 1 1/2 weeks so far, and hopefully it will stay through the summer. Could it mate with an American Goldfinch?
I believe its an introduced species that is locally common in on the eastern sea border. I would doubt it would mate with an american goldfinch as there are probably other European Goldfinches local to you. However they are from the same genus and if it is known that Carduelis finches have hybridized but this is not a regular occurence.
All I can say is, introduced or not, I would love to see one of these goldfinches at my feeder. I envy you fisher