I found a good explanation in case someone else struggled with this:
In Greek, the word "τι" (ti) is used to ask for specific information, while "ποιο" (poio) is used to ask for general information or to ask for a choice between options. In the case of asking about the color of something, "ποιο" is the correct word to use. The correct way to ask about the color of an apple in Greek would be "Ποιο είναι το χρώμα του μήλου;" (Poio einai to chróma tou mílou?). To ask about the color of shoes, you can use the same construction, but with the word "παπούτσια" (papútsia) instead of "μήλου" (mílou): "Ποιο είναι το χρώμα των παπούτσιων;" (Poio einai to chróma ton papútsion?).
In the sentence "Τι χρώμα παπούτσια φοράνε;" (Ti chróma papútsia fórane?), the word "τι" (ti) is used to ask for specific information about the color of the shoes that someone is wearing. In this case, "τι" is used because it is asking for a specific color, rather than asking for a general description or a choice between options. For example, if someone asked you "Τι χρώμα παπούτσια φοράς;" (Ti chróma papútsia forás?), you could respond with a specific color, such as "Φοράω μπλε παπούτσια." (Foráo mple papútsia.).