In a way, Danaerys donning red could just be an intimidation tactic so that the Starks and their allies will accept her as their new queen. But it can also show Khaleesi's state of mind. It could be that she's angry and passionate about defeating the White Walkers after they killed Viserion, or perhaps it shows that she has fully warmed up to Jon Snow and is now madly in love with him. Let's not dismiss the possibility that it could be both.
Another theory is that Daenerys is embracing her destiny as Azor Ahai. To recap, the red priestess (no, not Melisandre, we mean the other one) believed that Daenerys is the one who was promised. Many in the fandom believed it to be true as the Khaleesi was born from smoke and salt, has woken the dragons and will eventually bring the Lightbringer to destroy "The Others." However, there's also a discussion that Jon Snow is Azor Ahai and Daenerys will be the one to be sacrificed for the Lightbringer to be forged. And so, could the red outfit mean that the blood of Dany will flow on the ground soon?
But then again, we could all be overthinking and costume designer Michele Clapton just wanted to dress Dany in red as a homage to her Targaryen heritage. In a few weeks, we'll know which of these speculations are true. She played the game of thrones, now will she win or will she die?
(Cover photo from: @gameofthrones)