It's possible to define color and background for text the user selects. And yes, the double colon :: is required.

Try it below. If you select something and it looks like this, your browser supports selection styles.

This is the paragraph with normal ::selection.

This is the paragraph with ::-moz-selection.

Testsheet:

p.normal::selection {
				background:#cc0000;
				color:#fff;
			}

			p.moz::-moz-selection {
				background:#cc0000;
				color:#fff;
			}