The green-morph Pine Siskin (top, right), normal Pine Siskin (bottom, right), American Goldfinches (on left.)
Note the big, yellow, greater coverts on the wing and yellow at the base of the primary feathers on the green-morph Pine Siskin at the top.
Here you can see that the undertail coverts of the green-morph Pine Siskin on the right are yellow, not white.
A very rare Pine Siskin has shown up at our bird feeder, one that is considerably more yellow than the other Pine Siskins. A small percentage of male siskins can show an abnormal amount of yellow in their plumage and appear greenish on the back. Scientists examining museum specimens found that only 1% had this coloring. These are called "green morphs" of the Pine Siskin. They have more yellow on wing, side of nape, flank and often undertail coverts and the backs may appear greenish. Not much is known about the green-morph, but it is not considered a subspecies difference.
In Pine Siskins in general, males have more yellow on them than females, but this varies by subspecies. The sexes cannot always be accurately identified in the field.
The Eurasian Siskin, quite a rarity in the U.S., can be confused with the green-morph Pine Siskin, especially the female Eurasian Siskin. You can see a photo of the female Eurasian Siskin Here. The male Eurasian Siskin has a black crown and bib and is not as easily confused. The Eurasian Siskin is an Eurasian species, and is accidental in spring in the outher Aleutians. There are records from northeastern N.A. from ME, MA, NJ, NF, etc. but there are questions as to whether these were wild or escaped birds.
There is a very informative paper on "Eurasian Siskins in North America—distinguishing females from green-morph Pine Siskins," in American Birds, 1989, which spells out the differences and says,
"The large yellow tail and wing patches that may first draw attention to a green-morph Pine Sickin at the same time eliminates the possibility of female Eurasian Siskin. In contrast, real female Eurasian Siskins are not so brightly marked and could easily be overlooked among Pine Siskins....Yellow undertail coverts, if present, eliminate female Eurasian Siskin.
Since our bird had yellow undertail coverts, that would make it a green-morph Pine Siskin.
You can downolad a PDF of the article by googling this -