Learn how to set the background color for selected text (instead of the entire document) in TextEdit ...

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.

Make sure the color doesn't interfere with the label's readability. If the color is dark or saturated, consider white text or a complementary color, and consider making the text bold and outlined.