Mac OS X will render the images using the color profile they got stored, actually png gamma correction would be more exact. This is not immediately apparent because at a glance the files may look identical but using the pngs over a CSS background color often reveals an unpleasant sight on the mac, especially on Safari.
To prevent such a color mismatch either use a different image format or strip the png gamma correction. In the case of the latter, pngcrush does a pretty good job.
I was pleasantly surprised that I could install it with ease on Fedora ("yum install -y pngcrush"). To use it run:
pngcrush -rem gAMA -rem cHRM -rem iCCP -rem sRGB png-file-name optimized-png-file-name
http://www.viseztrance.com/2009/02/fixing-the-png-color-mismatch-on-mac-os-x.html
