Green Winged Teal

Here in Thamesmead, everyone is really excited by the rare sighting of a Green Winged Teal. Here is a picture of it taken this morning at the Crossness Outfall. Sitting with an ordinary Teal so you can compare the two

I managed to get a shot of it in the air but when I got a good look at the picture I had taken, I got a surprise. See below

Now this isn't the best picture ever taken but you can clearly see that it has a blue flash on the wing, not a green one. Any suggestions as to why?

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