Colours on a mac browser

I set up my BP Social theme options to include an image/logo for my site on the header.

The image is made in a specific “hex” colour and I have set the header of the theme to the same “hex” colour, so that the header and the image backround appear seemless.

I have viewed this on my windows laptop with various browsers and it looks fine, however I saw it using a Mac this week and the you can clearly see that the header and the image are two different colours.

How can this be?