I discovered another problem while developing a site, I was surprised that the colors rendered from a .PNG image was different between IE and Firefox!
Turns out this was because of Alpha Transparency Levels and Gamma Information that is stored inside PNGs.
Your header background is a .gif, whereas your logo is a png.
PNGs have alpha transparency levels, where as GIF files have only one level of transparency. Internet Explorer 6 and below don’t have support for those alpha levels of transparency.
Internet Explorer 7 supports these levels of transparency. So why is it that PNG files look darker then? This has nothing to do with transparency. This has everything to do with Photoshop. Photoshop stores gamma information in all PNG files, that causes them to be rendered differently in IE7.
Delete that gAMA entry out of your PNG files, and it’ll all look fine. You can use a freeware program, PngCrush, to do this: http://pmt.sourceforge.net/pngcrush/
Do a search on ‘PNG color shifts’, you’ll come up with a lot of hits.
http://www.polarbearlamps.net/2007/04/png_color_mismatch_on_the_web.html
http://www.molly.com/2007/11/20/your-best-practices-for-creating-great-pngs/
Taken from: http://www.lunarforums.com/lunarpages_message_boardsforumsfantastico_scripts/logo_background_color_different_in_firefox_and_ie-t46125.0.html