The Mac and the PC use slighly different color palettes. There is a 216 "browser safe" color palette that both platforms support; the Microsoft color picker page has some good information and links to other resources about this. In addition, the two platforms use different gamma (brightness) values, so a graphic that looks fine on the Mac may look too dark on the PC. The only way to address this problem is to tweak the brightness of your image so that it looks acceptable on both platforms.