I remember reading this in a college English class about 20 years ago. The elements I can remember are:

1. Cinderella's mother was killed by the king because she was a witch.


2. Both Cinderella and her mother had red hair, this was a sign that they were witches.


3. Cinderella used ashes in her hair to hide the fact that it was red.


4. Cinderella planned to kill the prince in order to hurt the king. (Revenge)


5. Midnight is significant in that it is the witching hour.

"When the Clock Strikes" (1980) by Tanith Lee

It was first published in Weird Tales #1.

The Duke had come to power by ridding himself of the members of a rival house. Yet one member of that house remained—a woman who attempted to use her sorcery to get her revenge. When the woman dies, her daughter, who calls herself Ashella, continues her fight against the Duke, and his son the Prince.

"Cinderella's mother was killed by the king because she was a witch."

"Listen to me, my daughter," she cried, "and listen carefully, for the minutes are short. If you do as I tell you, you can escape their wrath and only I need die. And if you live I am satisfied, for you can carry on my labour after me. My vengeance I shall leave you, and my witchcraft to exact it by. Indeed, I promise you stronger powers than mine. I will beg my lord Satanas for it, and he will not deny me, for he is just, in his fashion, and I have served him will. Now will you attend?"

"I will," said the girl.

So the woman advised her, and swore her to the fellowship of Hell. And the woman forced the knife into her own heart and dropped dead on the floor of the tower.

_{"When the Clock Strikes"}

"Both Cinderella and her mother had red hair, this was a sign that they were witches."

The woman caught the girl by her red hair and shook her.

_{"When the Clock Strikes"}

"Cinderella used ashes in her hair to hide the fact that it was red."

Do you suppose she smiled secretly as she said this? One might suppose it. Presently she donned a robe of sackcloth and poured ashes over her red-copper hair. "It is my penance," she said. "I am glad to atone for my sins."

_{"When the Clock Strikes"}

"Midnight is significant in that it is the witching hour."

And now it is almost midnight, the moment when the page of the night turns over into day. Almost midnight, the hour when the figure of Death strikes the golden bell of the clock. And what will happen when the clock strikes? Your face announces that you know. Be patient; let us see if you do.

_{"When the Clock Strikes"}

Found with the query short story cinderella withch red hair ashes.