"Yesterday, I bought a green skirt."
Translation:Χθες αγόρασα μία πράσινη φούστα.
No, the presence of an adjective is neither here nor there. The indefinite in Greek is closer to an English "one" than the English "a". In a sentence like this, I think it'd be better to include the indefinite, because you could have bought two skirts - the μία specifies that you bought only one skirt, not two or three. But, if you just wanted to make a statement about how you want to buy a green skirt, in general, you could drop it - θέλω να αγοράσω πράσινη φούστα.
The difference is a bit clearer in a sentence like "I am an author". In Greek, if you said "είμαι συγγραφέας", that's a pretty close translation of the English. But if you said "είμαι ένας συγγραφέας", in English, that's a bit more like saying "I am one author", the implication being that there are other authors too, "I am one author (amongst many)".
TL;DR: My understanding is that the adjective doesn't affect indefinite article use.