On my monitor I can see some green on the wing patch but it is in shadow, so may appear darker or even reflecting some blue sky but you are right, it doesn't have that distinct green color. Sibley's lists a "dark line across cheek" as a field mark for adult, female GWT. I'm no expert so let's see what they have to say.
Female Green-winged Teal was my first thought too. Shade and indirect lighting often play tricks on speculums' colours. This is the smallest North American dabbler, did you notice that is was smaller than the other species you mention?
Re-reading my Sibley's I do see the mention of the cheek line althought the illustration does not feature it prominently. This bird was smaller than the other ducks in the vicinity. Certainly seems to point to GwT, but looking at the speculum on the original image it looks blue to me. I suppose it could be a reflective effect kind of like the way a male Mallards head can seem to change color as the lighting changes.
The speculum is tricky, but then all reflective feathers can be tricky. In fact, I don't think the blue looking feathers are even the same feathers that show as green on a Green-winged Teal. Take a look at these spread wings of Green-winged Teal on the USGS site. Notice how the strongest buff in all ages seems to be above the dark part of the speculum, not the green part of the speculum. Looking at this and looking at the buff edge showing on your bird makes me think the green is tucked under.
The line on the cheek if fine for Green-winged.
Two obvious marks that knock Mallard out of contention are the all dark bill and the dark legs. The bill on a female Mallard should be orange with a dark saddle on top. The legs should be bright orange.
Another mark against Mallard is the pattern of the scapulars. These are the feathers just above the wing with the last one actually touching the speculum. (Click this link for a good bird topography diagram.) On a Mallard, they always look kind of "tigery" to me. Looking closely at some photos on the web, the reason for that look is that the feathers have pale centers, dark outers, and pale edges making them look almost striped. I think I'm fumbling my description on this, but if you do a Google image search for "female mallard" it should make sense. The pattern on your bird looks fine for Green-winged Teal.
If any of that is more confusing than enlightening, don't hesitate to give me a smack.
Mallard?
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