In Wolfgang Petersen’s Troy the story is woven around the conflict between Agamemnon’s combined Greek invasion and the Trojans defending their home. As the first Greek ships land on the beach, we can plainly see Achilles and the Myrmidons sailing under a black sail and fighting in completely black armor. After the remaining Greek soldiers land and disembark as well, we can see the theme of black armor is not limited to the myrmidons, but to all the soldiers fighting for Agamemnon. The added component to this theme of black is that the armor is a dull black, no luster, shine or sparkle is seen on any Greek soldier throughout the film.
Eudoros, Myrmidon second-in-command, attacks a Trojan soldier
As established by classic literature, the color black has thematically symbolized evil, death, destruction and war. Petersen uses this symbol to reinforce the idea that the Greeks are invading another land and acting in what we would consider an “evil” manner. He suggest this to the audience through a combination of actions, dialogue and symbols. But the tool used to tie together all of these different messages is color, which constantly reminds the audience what viewpoint they should take on the Greeks every time they are on screen in their armor.
Hector is surrounded by Myrmidons
This is contrasted by the Trojan soldiers and their resilient gold armor with blue tunics. Thematically these colors tend to symbolize honor, respect, intelligence, vigor, fluidity and wealth. Their armor shines in the bright sunlight and is ornate on the chest plate and the bracers. Taken alone, this armor would symbolize great wealth and intelligence, descriptions that could apply to those an audience should root for or an audience should root against.
But when compared to the armor of the Greeks, Petersen makes it readily apparent which side is the morally superior one and who the audience should side with. This allows him to first establish the primary villains and heroes of the conflict by what they wear. Petersen can then build upon this base understanding, by adding an individual’s dialogue and actions to their character to make them whole.
On the Trojan side Hector and Paris wear sterling gold armor and are generally positive in their words and actions, making them heroes. Ajax and Agamemnon wear black armor and have very few if any positive words or actions, making them villains. Then there are characters like Achilles and Odysseus, who wear he black armor of a villain but whose actions and words are those of a hero, making them ambiguous.
By using color to create themes and help establish characters, Wolfgang Petersen creates a narrative rich with with deep characters that the audience can cheer for, against or remain undecided.
Antiquity and Revenge on Screen 
Another aspect of Peterson’s theme of color is the comparison of Sparta and Troy’s normal tunics. At the beginning of the film when Hector and the Trojans travelled to Sparta to negotiate and then celebrate peace, the scene is filled with red/maroon togas. Once returning to Troy, however, there is an obvious shift in tunic color to blue, as you mentioned. Thematically, red, as the color of blood, tends to symbolize war, anger, and violence; whereas, blue is a more peaceful color. This contrast between tunic colors and their connotations reflects the differences between the leaders of the two societies. Agamemnon, the grand leader of Sparta and the surrounding areas, is extremely hungry for more land and with it, more power. This greed is his primary reason for going to war with Troy but was sparked by the kidnapping of his brother’s wife, Helen. Priam and Hector, on the other hand, are extremely devoted to their family and the people of Troy. They fight Sparta with honor protecting the ones they love whether it’s Paris or the Trojan citizens. So not only did Peterson use armor color to differentiate the good and bad guys in the film, but he also used the color of their casual wear, to a lesser extent, to contrast the leaders and their motivation for war.