A few characters of mine have eyes that change color or such depending on different elements.

Nashoba's normally light pink eyes turn bright gray when he uses his metal manipulation.

The color drains from Yepa's irises when she uses any of her magic.

Ethan's eyes glaze over an iridescent flickering rainbow when he sees visions or is in a vision-trance.

Ahmik's eyes revert back to their original dark purple if he has used too much magic or is in the presence of Donnie and his brother is consciously pulling enough magic from him.
They also have faint icicle markings within them if one dares to get close enough to see.

Chris' light red eyes get ever darker the more bloodlust he experiences.

Eruera's eyes are actually a blueish-brown hazel but his totem's ability makes them black.

Yaotl's eyes turn/turned a glowing dark red when he uses:used his hemokinesis.

Not only do his eyes but the scar on A'Ciliz's left eye glows yellow as well when he performs magic.
Though he can make it to where simply his pupil glows instead if he wanted to.

Ammon's right eye is always a light glowing blue, while his left eye is as both used to be, the original color, a dark gold.

Nāwai's eyes always seem to ripple as if the disturbed surface of water. And even more so if he feels a strong enough wave of emotion such as happiness, anger, sadness, etc.
(Pun intended~)