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.

One way to customize labels is by adding an eye-catching background color to them. You can change the color of the entire page using a solid color, a gradient or even a picture using Publisher's ...

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.