It tends to be the other way round at least intially. Lessers fade from late winter and their plumage can become very Mealy like by spring. Later on, in the summer, as all the pale feather edges wear off, both, but particularly Lesser, can look rather dark. After the post breeding moult both initially have a lot of buff and brown in there plumage until the feather fringes and wingbars become bleached. Mealies are perhaps most easy to distinguish between say October and February.
The big problem with Coue's and Mealy is that no single feature holds. ID is based on the probability of a given bird showing a suite of more likely features. Both can have streaked or unstreaked rumps (though the latter is much commoner in Coue's) or at least appear to in the field, both have variable streaking on the flanks and undertail coverts (and even worn Lessers can appear to have un-streaked undertail coverts), variably marked ear coverts etc. Assessing 'fluffyness' of plumage also means assessing weather conditions and what other nearby redpolls are doing. Bill size is variable and an assessment of the position of the forehead feathering (sleaked or raised?) is required. A 'classic' Coue's with a big white rump, minimal streaking, small looking bill etc. can be very distinctive but these 'classic' birds may well not be the commonest plumage variation particularly 'out of range' birds where a higher proportion of young birds is likely.
an image so poor that you can see anything you want!
