What you want to do is a apply a different "skin" to that page.
Presumably, you don't want to change the page layout, just a background color. To do this:
- Make a copy of the skin file, and paste it back with a slightly different name, e.g. myskin-redbackground.htm or myskin-rebackground.ascx.
- Edit the skin file and locate the style that provides the background color. Change the name of the style to, something like myBackground.
- Edit the corresponding skin.css file to add a style that corresponds to myBackground and add the background color reference.
Make sure that the updated skin.css file and the new skin file are in the same directory as the skin you are using. You will need to go to the Skins admin page, select the skin, and "Parse the skin package" if you are using the .htm version of the skin.
Then, go to the page, go to Page Settings, and select the skin.
If this doesn't make sense, or seems overly complicated, you'll need to learn a bit about DotNetNuke skins. Armed with a little bit of knowledge, this shouldn't take more than an hour (probably considerably less) of work.