1. When you remove background image, the selected background color is displayed.
Go to Appearance > Customize
Expand Colors section
Click Select color
Then click on Default button
If still issue, please share your website url for more targeted support.
2. Such type of advertisement display will require fair amount of custom coding. You can try these plugins for such functionality:
http://codecanyon.net/item/wp-background-takeover-advertisements/8759542