"Your shirt is black."
Translation:Το πουκάμισό σου είναι μαύρο.
Why does Duolingo insist on omiting accents in cases like this? It should be πουκάμισό σου, shouldn't it?
Yes, it should. The contributors are aware of this, once they are allowed to edit the (currently locked) Greek sentences they''ll fix this. Please remember that the course is in beta, mistakes and omissions are to be expected during this phase. :) But it's good that you're mentioning it as more users who might not know about this rule will see it.
That's correct, but something to be aware of is that when Greeks are being polite or cordial (particularly with a stranger) they may say σας instead of σου (even in the singular instance)