Your #main-header background is actually set to orange, but is overridden by the background image that includes the stripes and the pink.
The image is not loading in IE, so the default orange is showing instead. As a fallback, you may as well change that background to pink.
The reason it's not showing in IE is because of the filter on #main-header. Those damned things cause so much trouble!
ah, I have an options panel which allows me to choose my colour scheme so I've chosen blue and taken out the colours and replaced it with my background colour. I didn't realise there was a #main-header as well. Will it work if I remove the code in the #main-header and replace it with my bg img?