This is a strange one...I have a customer for whom background colors don't show up in either Firefox or IE. For example, check out msn.com. See how theres a blue background? For my client, that background shows as white.
Any thoughts? I couldn't find any settings to affect this issue.
On Firefox there is a Checkbox for "Use System Colors" also for "Allow pages to choose their own colors..." under Tools|Options|Content|Fonts and Colors|Colors.
In IE it's under File|Properties|InternetOptions|Colors.
Similar to @Laura's answer... some sites do not specify a "white" background on the body tag, since "most" browsers default to that (since the Netscape 2.x era which was gray) However that "default" is provided by windows... (or your given OS) thus if the user changes their default to gray, or black, or whatever, that is what will appear.