Your duck looks great!
While in reality ducks can dive, swim, and fly, what matters for 0 A.D. is their walking animation, like the chicken or peacock, because otherwise units can't catch and kill them for food, which is the main purpose of animals in game. Unfortunately amphibious entities are not (yet) possible, it would also be great to have for e.g. crocodiles and hippopotamuses.
As for colour variation (i.e. textures), the one with the green head, brown chest, and light body is most iconic, but those are always adult males:
Females are brown:
As are young adults, both female and male (you can see they're young because their feathers are not fully grown):
Ducklings have a different colour pattern and are covered in down:
However, there are also entirely white ducks:
And many colour variations in between:
Finally, where in the world mallards live:
Given the nature of 0 A.D., I don't think it's worth it creating ducklings or young adults; having just one mallard shape + animations is good enough. For textures, I'd recommend having at least three: brown, multi-coloured, and entirely white, in a 3:2:1 ratio. More texture variation would be welcome, but is not a priority, and could always be added in the future.