How do I add a custom style that can be applied to anchor tags?

I want to let users style the links they create in the rich text editor. How do I add a custom style to the ribbon for this? I've tried:

.ms-rteStyle-fancyAnchor
{
    -ms-name: "Fancy Anchor";
    color: purple;
}

and

a.ms-rteElement-fancyAnchor
{
    -ms-name: "Fancy Anchor";
    color: purple;
}