The “Named Colors” section of the CSS Color Module Level 4—the latest ... e.g., #FFFF00) or an RGB color code (e.g., 255,255,0). These options allow a greater degree of choice and precision ...

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.