Do you mean the highlight color when you left-click on text and drag the mouse?
If that's the case, those colors are determined by the user's system preferences, not by CSS. I don't know of any way you can change that with CSS.
If you're talking about changing the background color of a link, here's how to do it.
The :hover is a psuedo-class for the anchor tag, for when the mouse hovers over the link.
You could also apply that background-color: color; property to almost any HTML tag.
If that's not what you were aiming for, my apologies.