For me, and I do both eyes and hair, color changes are symbolic of magical power output.
For example, in chapter one Roxanne’s hair is brown at the root but gradually becomes lighter to an orangey color at the ends. Normally, when not using magic her hair is brown. Her eyes are also brown.
When she uses her powers for the first time, her hair becomes more blonde and glowy. Her eyes change into yellow stars and they also glow.
Once her magical power levels out, her hair returns to brown-orange and her eyes lose the star shape but they remain yellow. They remain yellow to show she is more powerful now, thanks to accessing her power. Once the “Witching Hour” is up, she returns to her normal state and has brown hair and eyes again.
With Cassandra, in her first appearance her hair is all black, implying that at this point in time she’s not putting forth any effort in generating magic. However in chapter 2 you see her hair has some orange in it and it glows at one point. This, plus Roxanne’s case, is meant to show magical power is linked to emotions - Cassandra’s hair started glowing because she was angry. Roxanne unlocked her power, and glowing hair, because she instinctively wanted to protect her friend.
That said it’s not always this case. Priscilla has green eyes when you first see her, but her eyes are really blue. Her hair remains blonde in both cases, implying she’s already unlocked her powers but isn’t generating much output enough to change her hair. With Hazel, her eyes are pink (while her natural eye color has yet to be shown) and she has brown hair with pink streaks. Hazel is very emotional so the eyes show she’s unlocked her power while her hair shows her always generating some magical output because she’s so emotional.
Annika is an odd case. Her eyes are naturally light blue, but the underside of her hair, and the hair halo / light streaks, are a light purple and is otherwise black. In Annika’s case she actively suppresses her magical power for spoilery reasons. So her hair shows she’s generating magical power but even tho she’s already unlocked her abilities her eyes remain blue - they should be purple.
Realistically her hair is mostly done for my aesthetic reasons, which was inspired by a character from Kampfer. But still.
Eventually I’ll show that hair magic hair color isn’t always tied to emotion and that the more experienced a witch gets the more they can control it. And for reasons hair color changes can also be a warning sign like in nature when some animals display themselves. Like how poisonous animals can be bright and colorful, you know? When a witch starts glowing or their hair changes, it’s like a cobra displaying their hood to intimidate or display that they will attack.
There’s a reason for that but spoilers.
Regardless they always revert to their original non-magic colors when they can’t maintain their magical forms anymore.
Yeah I put a lot of thought into the color display of witches. But also sometimes you can’t tell with some witches cuz their natural hair color matches their magic one. Like Melanie has black hair normally and her magic color is black. Her secondary color is red so I could use that but probably not.