Can I change the background color of posts ?

Fourteen Colors retains the color patterns of Twenty Fourteen. Since the white content area is fundamental to the theme’s design, and is appropriate for the vast majority of color schemes one could apply to Twenty Fourteen, it can’t be changed with the plugin. You’ll need to use a fair bit of custom css for that, or there may be a child theme that does it, but you may want to consider using a different theme entirely.

Thanks Nick, The client agreed to the regular white background