Exotic engrams do not drop. They are only available from Xur.
As the other poster alluded to, green engrams can be shaded by red lighting effects, such as the death screen, to look yellow.
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Trust me I'm a farmer, lol, exotic engrams do drop.
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And, for some backup... See the BNGHelp post in the link below. http://www.bungie.net/en/Forum/Post/70989395/0/0
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Exotic items drop. But bungie has stated on these forums, multiple times, that exotic [i]engrams[/i] do not drop.