I did this on the stove top with hot, not boiling water, with about 1/2 cup vinegar. I just threw the shells in and filled enough water to cover them about 1/2 inch and poured in some dye. I didn’t measure any of it.
I stirred them around until desired color and then put them on a few paper towels to dry.
It was a simple and fast project I did while bored. I am also including a photo of my favorite mermaid sitting next to some natural sea shells.