The Koloa or Hawaiian Duck looks very like the female Mallard to which it is related and which threatens its survival by interbreeding (exacerbated by hunting and habitat loss).
The Koloa has a teal-green speculum instead of the Mallard blue. Males (above) have a darker head and a grey-olive with black bill and yellow-orange legs/feet.
Male and female have similar plumage
Females (above) have a mallard-like head, an orange with black bill and dark orange legs/feet.