Showing modal window as you want, setting with color, style, and wp's conditional branches.
This plugin shows modal window using jQuery simplemodal plugin.
While it's activated, a necessary JS and Stylesheet is be already inserted WordPress's section. And You can set the Content, Styles, Conditional branches(e.g. only showing frontpage, and 404 page.) from the management interface added to the submenu of Settings.
Usage example: Only shows in front page - Display New item, band's next gig, or Ad banners. Shows in specific page - When the content of the page is updated, displaying important appendix about difference. Shows in specific post,and 404 page - Maintenance information alerting.