Nothing to do with Mac vs. PC, well, at least not directly. Same old, same old: Read up on color management and how to use it and tell whoever is reviewing this to do the same. No point in tweaking colors on one system when the person on the otehr end doesn't care to calibrate his system as well.
+1 to Mylenium's comment.
It's all about color-management, and Macs can do it every bit as well as PCs.
also, you are probably going to look like the "problem" until the people down stream figure out why their color is off
but here is my advice:
hardware calibrate/profile your monitor and check it with a known reference image like the PDI Targets so you are sure what you are looking at
be sure you Edit> Convert to Profile: sRGB (if not already there)
and Save As with Embed Profile option checked
at that point - if they are having color issues - their monitors are most likely out of calibration (or possibly they have wide-gamut monitors)...
also if they are viewing your color in Photoshop, they could have their Edit> Color Settings> Color Management Policies set up to ignore profiles and 'assume' their working RGB -- hard to say with out knowing where they are seeing the problem
also Photoshop View> Info pallet can sample areas for detail and tone in areas they say is off