"Your shirts are black."
Translation:Deine Hemden sind schwarz.
Possible German translations for "Your shirts are black":
Deine Hemden sind schwarz - (I'm talking to one person who I'm familiar with)
Eure Hemden sind schwarz - (I'm talking to multiple people who I'm familiar with)
Ihre Hemden sind schwarz - (I'm talking to any number of people who I'm formal with)
Duolingo should accept all three German sentences as a translation from the English, because we don't know the context.
It could be. Deine for one person, eure for multiple. The english sentence doesn't clarify this context, so you may report it.