Answer
It is permissible to wear a green Imaamah, however, it is not Sunnat.
Allaamah Suyyooti (RA) has mentioned in his Kitaab Risaalat Zarnabiyya that
a green Imaamah is not a Sunnah. (Addi’aama pg.99)
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai