Just figured out what your problem is. IE 6 doesn't have support for png file images. What then happens is it greys out the transparency on a web page, which is contained in that image for your background. Hence making it lighter. Either gif or jpg it up or use the png hack for ie:-
http://snipplr.com/view/1179/png-ie-fix/
If I were you, just gif it and make a repeating horizontal image.
But of course, your designer should have already known that.
