From left to right, Mealy Redpoll, Coue's Arctic Redpoll and Lesser Redpoll. All of these birds were originally on the same photo, I've just moved the Lesser a bit closer to the other two for better comparison. Notice how pale the bird I'm calling Coue's is compared to the Lesser, especially on its back and flanks. There is hardly any brown in the Coue's. Furthermore, its flanks are clearly white and not the buff colour of the Mealy and Lesser. I've attached a series of photos below which all show the Mealy and the Coue's, and on every photo you can see that the Coue's is clearly paler with white flanks and probably rump, illustrating that the white effect on the first photo is not just a one off trick of the camera, it's consistent through lots of other photos of the bird. Also, look at the white mark under the eye of the Coue's and the bright white wing bar, and then compare with other photos available on the internet of the bird. Finally, check out the small bill of the Coue's compared to the Mealy.
Lee Evans has done an excellent write up of Arctic Redpoll identification here. A couple of quotes from that guide include [Exilipes (Coue's)] has "predominantly gleaming white undertail coverts" and "gleaming white rump, with some occasional streaking at the sides and at each end". The rump is not really visible on any of my photos, but clearly the area around the undertail is gleaming white. Lee goes onto say "Mealies often show white ‘tram-lines’ but do not have such intense white ground colouration". I believe that my photos do show more than just tramlines, the ground colour is clearly white. Also note that if you look at the second photo on Lee's guide, you will see that although Coue's has an extensive white rump, it is not always visible when the birds wings are closed (see second photo down).
