Why Alicent Hightower is wearing a green dress

At Rhaenyra's wedding, Alicent comes to the top of the stairs in a green dress, interrupting the king's speech and attracting attention to herself.

Larys: “The beacon on the High Tower. Do you know what color it glows when Oldtown calls it banners to war?” – “Green?” That is, the people of the Reach go to war like children.

In cinema, color has a meaning and is a symbol of something.

it is generally accepted that the green color symbolizes nature, development, renewal, youth, childhood, children, immaturity, inexperience, unpreparedness, untrained, etc.

Alicent, wearing the green dress, shows those present that she declares war on Rhaenyra as a pretender to the throne.

From that moment on, Alicent is shown in green clothes, which symbolizes her childishness, inexperience, immaturity. Just as in her youth, Alicent argues with her father, prevents him from fighting Rhaenyra's camp, acts contrary in a futile attempt to reconcile everyone.

And Alicent also reacts to the tricks of Larys like a child. Out of fear and helplessness, making an immature decision to use his help.

In green, the authors show that she never became a real queen, but remained a child. Perhaps because, as queen, Alicent is out of place.