You don't have to use "label--". So that this won't be complicated for you, try doing this. Or just do this if label just isn't really working for you.
Create a new css class with the same attribute as .label. In this example, badge.
.badge {
background: #000;
color: #fff;
border: 1px solid red;
}
And then the next ones will be your customs. So if you're doing Administrator.
.Administrator {
background: red;
color: yellow;
border-color: pink;
}
In your usergroups, just put "badge Administrator". Without the quotes. That should show up. If it won't show up, hard refresh your browser. It could be just a cache issue.