The "cinder" part of the name "Cinderella" means that the girl Ella was covered in cinders, or ash, from the fireplace. This made sense for someone confined to cooking and cleaning, particularly cleaning the fireplace itself. Cinderella would be covered with this dark material and would stand out quite distinctively.
Earlier versions of the story tried to describe her as looking like a shadow due to the dark color. However, "Shadow Ella" was not easy to say and "Umbra Ella" just confused people.