Add color to boxes

Hi all, I want to add color to the inside of a box. I understand how to use tick marks to create a boxed line around some text but how do you add color to the inside of the box? GitLab uses these colored boxes in their Markdown guide but doesn’t tell users how to do it! Makes no sense. I can’t find the info anywhere else.

I just want to add black or some other color to the box, like this: