WordPress' built-in editor has an HTML mode that lets you override the background color of a post by changing its CSS (Cascading Style Sheet) background-color attribute to the color of your choice.

How to Change the Background Color in a Microsoft Publisher Label. Using Microsoft Publisher allows you to create many types of labels. You can perform mail merges to send newsletters to customers ...

To change the background color, drag a color swatch from the color picker and drop it on the window. Repeat as necessary with varying colors until you have one you like.