"It is her dark green dress."
Translation:Det er hendes mørkegrønne kjole.
I asked my Danish friend: its because if the subject is "it" you always use det
I also had this question. The best answer I've found so far is here:
When the noun is definite (e.g. her dress/the dress/that dress...) the adjective has the plural/definite form, which often has a -e ending.
En mørkegrøn kjole
Hendes mørkegrønne kjole
Thanks so much! That explains it perfectly. I've been wondering about this for months.