It's got nothing to do with Windows vs. Mac - walk into any office and look at the different monitors on folks' desks. Assuming you're using a standard color scheme (sRGB, etc.) the information will go out to each of those monitors the same way (i.e., white = "ffffff" which is hexidecimal for "turn the red, green, and blue values for that pixel all the way up"), but that "white" screen is going to look different on each desk's monitor (unless they've been calibrated).
If a certain monitor is calibrated differently it's going to look different, and that's got nothing to do with OS, web browser, etc. If you really, really want everyone's screen to look the same you'll have to drive around to their house and calibrate everyone's screens individually.
The best you can do is to use a standard color scheme (I like sRGB) and hope for the best. Don't forget - most users won't be staring at the same web page on two different screens so they probably won't notice the difference.