The YellowBox processor places pre-formated text into a colored box, which is indented to offset it from other text on the page.

If the first line of the content consists of the string 'color=' followed by a color name or code, then that color will be used for the box background instead of light yellow. The color name can be one of the standard web color names, or a 6-digit hex code, preceded by '#'.

For example:

{{{#!YellowBox
  This is pre-formatted
    text
}} }

will be shown on a yellow background, like so:

  This is pre-formatted
    text

---

{{{#!YellowBox
color=#ff8080
  This markup is ''not''
    interpreted.
}} }

  This markup is ''not''
    interpreted.

should show on a red background.

---

{{{#!YellowBox
color=lightskyblue
  Such a pretty background
color
}} }

  Such a pretty background
color

should show on a Light Sky Blue background.

See https://www.w3schools.com/colors/colors_names.asp for color names.