This species really stands out in the winter with their bright yellow underparts and cold greenish backs. Note the white wing bar and the little white patch at the base of the primaries. The more common American Goldfinch has a warmer golden brown cast in winter, and has buffy, not white, wing bars.
Pale females, like the top bird, are harder to distinguish from female American Goldfinches, as they lack the bright yellow underparts. Note the cold greenish back (not golden brown) and the white (not buffy) wing bars.