To get back to the original question, Morwen, I think Marie Wells is a mezzo-soprano. Voice type (soprano, also, etc) is based on how high and low one can go.

Lightness and darkness have nothing to do with what kind of voice one has. The same singer can sing light or dark, depending on how she shapes her throat. (I learned how to do this from one demonstration by a choral conductor.)

In opera it used to be a tradition that altos sang dark. This is no longer fashionable.

We speakers of English are great ones for nouns. We say that a person has a "voice." But we don't actually have VOICES. What we do is SING (verb)in a certain way. I bet that around the house, Marie Wells could sing soft & bluesy, sing hard & angry or sing a delicate lullaby. She would still sound like herself, but each kind of song would have a different timbre.