Disclaimer: I don't own Soul Eater.
His eyes were red, but they seemed to shift colors depending on his mood.
When she barged into his room in the morning, armed with a frying pan, his eyes would be bloodshot, the whites tinted pink. He'd rub them blearily, blink at her a few times, then ignore her and pull the covers back over his head.
A few seconds later, he'd find himself sailing out the window, sent flying by the frying pan.
When she made his favorite dinner, they would light up and she thought they looked like rubies, the way they sparkled as he smiled at her, before grinning hugely with his sharp teeth and devouring the meal whole.
When he played the piano, his eyes reminded her of red wine. They were dark, almost a deep purple, as he focused on the keys, but sometimes when he moved his head or glanced at her occasionally to gauge her expression, the light would reflect off his eyes and she could catch a glimpse of brilliant crimson before he looked back down at his hands again. Then she would feel drunk and heady, like she could drown in those dark crimson pools.
Sometimes, often during a particularly difficult mission, she would stumble and the demon would pounce, backhanding her viciously and sending her flying. Then he would switch back from his Weapon form and stand protectively in front of her. She couldn't see them, but then his eyes would look like dark pools of blood, savage and murderous as he bared his teeth at the demon, ready to tear apart anyone who harmed his meister.
But her favorite was during those times when they were by themselves, just talking about anything and everything, and he would laugh and smile. Then, she thought his eyes were like a fire, cozy and red, giving her warmth.