An excellent question indeed.
Noch keine Antwort...
Still remains a mystery to this day.
Its because the co-founder hates green. When designing the logo he told the team that he hated green.
So they thought it would be funny to make Duo green. And the rest is history....
Learnt this off Tim Ferriss' interview with Luis Von Ahn
Er ist grün, weil er grün ist.
Weil Duo die Natur liebt. oder Weil Duo Vegetarier ist.
duo isst zu viel gemuse
Asking the real questions !
Warum ist der Himmel blau?
Ja! Warum ist Duo grün?
Because of all the money he gets from plus members.
Why's "Why's Duo green" incorrect? It shouldn't be
Er ist zum Teil Papagei.