Do you mean allow HTML tags in a post? I think there's an option for that in the Forum Manager. But I think maybe the issue isn't just putting it in the db, but that there's lot's of HTML a user could write that would mess up the page.
You can set background color by something like:
HTML Code:
<span style="background-color:yellow">some text</span>
and while it might be tempting to make a bbcode that allows the color to be specified, I think if you do that there's nothing stopping a user from entering any style attributes (not just a color name), so they could do things like position something anywhere on the page.