Page background colour.

The background color option is available by default in Twenty Fourteen, along with the ability to set a background image (it is only visible on screens wider than 1260px). The background color option is displayed below the Contrast Color option, in the colors section of the Theme Customizer.

If you’re referring to the white color that is directly behind the actual site content, that cannot be changed with Fourteen Colors, because it is fundamental to the design patterns of Twenty Fourteen. If you’d like to change that you’ll need some fairly involved custom css.