"He has a shirt, but it is not red."
Translation:Αυτός έχει ένα πουκάμισο, αλλά δεν είναι κόκκινο.
Because the pronoun is actually implied. You are talking about the shirt, so you could actually just ommit αυτό, or εκείνο. It's not wrong, it's just not a necessity. :) Unlike english, because if you ever said "He has a shirt, but is not red" it doesn't sound nor is correct. xP