Is this _**[Jack the Giant Killer][1]**_ (1962)...?

> A farmboy-turned-knight must protect a princess from the schemes of an evil wizard.

The [Wikipedia page][2] explains the plot in detail, but the following paragraph in particular seems very close to your description, with an evil sorcerer named Pendragon sending demonic witches to capture Princess Elaine from a ship, and subsequently bewitching her to turn her into an evil witch. There's also mention of a mirror being smashed to free another female character, Lady Constance, from one of Pendragon's spells.

> Pendragon sends demonic witches to intercept the ship. Amid the chaos, the ship's captain is murdered, and Elaine is captured. Jack wants the ship to follow the kidnappers, but the crew refuses and cast Jack and Peter, the captain's young son, overboard. At his castle, Pendragon bewitches Elaine into an evil witch loyal to him; Pendragon returns to Cornwall and confronts King Mark. Pendragon tells the king he has one week to renounce his throne so that Pendragon can rule with Elaine by his side. If the king refuses, Elaine will be killed. After Pendragon vanishes, King Mark realizes Lady Constance betrayed him. Standing before a mirror, she appears in witch form. King Mark smashes the mirror, freeing Lady Constance from Pendragon's spell.

The following video clip shows Pendragon bewitching Elaine, causing her clothes to change and her eyes to turn yellow.

https://www.youtube.com/watch?v=mDquzEoyW88


  [1]: https://www.imdb.com/title/tt0056112/
  [2]: https://en.wikipedia.org/wiki/Jack_the_Giant_Killer_(1962_film)