Could this also be τι χρώμα έχει η φούστα; I've seen έχω when talking about what colour something is before...
Thanks for your answer teopap! So, another question: in English, there is, afaik, no difference between the two. Is there any nuance in Greek?
Ποιος/ποια/ποιο etc. implies that you are shown some specific colors and you have to choose which of these is the correct/you want/etc. With τι you are free to give any answer.
Example: a scene in a shop:
-Client: Γεια σας. Έχετε φούστες;
-Seller: Βεβαίως (=Sure).
-C: Τι χρώματα (=colors) έχετε;
-S: Μαύρες, κόκκινες και μπλε. Ποιο χρώμα θέλετε;
In English, I guess one assumes there is a finite set of colors to choose from, so "which" would be the most common usage. Different languages look at things differently. I think "which" should be an alternative here. I understand the difference in Greek, but I'd probably always use ποιο, since I'd use which in English, also in this situation.