Color problems on Mac OS X

I would guess that the background color is different because you have created it using an untagged image. The problem is because each application renders untagged image / color differently.

From Colour Inconsistancies in OS X:

In safari/ color picker, untagged image will render based on your monitor color-gamut.Iif you are using 2010 mbp 15, after proper calibration color-gamut should be close to srgb, but not exact match therefore the color variation.

You might want to read this website to better understand about untagged image in browsers. http://www.gballard.net/psd/go_live_page_profile/embeddedJPEGprofiles.html

In short, if you wish an image to render the same on all browsers, you should incorporate the color profile. Some older browsers may not take notice of it, but most modern browsers do.

The problem is explained clearly in the above embeddedJPEGprofiles link, with demonstrations and with advice how to incorporate color profiles in images.