You cannot make that work, post back is a server side event, usually one redirects out of postback after doing whatever processing is required in the post back so that there will be no duplicate postback if the user hits the refresh button on the browser. A LinkButton is more like a button than a link, colorbox can be used for links not linkbuttons, it is client side code (runs in the browser not the server) not server side and cannot be invoked from server side code.
To make a button look like a link in mojoPortal put CssClass="buttonlink". But you still cannot invoke colorbox from server side code.
Hope that helps,