(Smack upside the head! I just figured this out! )

This only works if your page or pasteboard already has a custom color. And it only works in Web Designer 7 and Designer 6.

To edit the pasteboard color which will be the area behind the page, open the Page and Layer Gallery. Unlock the Pasteboard Background layer. Click on the pasteboard area (which is now selectable) and edit the color in the Color Editor (Ctrl e). Lock the layer.

This only works with one page and so if you want to change all page backgrounds then it is best to create a Named Color and Ctrl drag and drop the color from the color palette.