A Mac and a PC handle colors differently. Graphics produced on a Mac and viewed on a Windows-based machine will appear darker than on the Mac. The opposite is true of graphics produced on Windows and viewed on a Mac. This has nothing to do with the SWF file itself; it has to do with gamma-correction differences between the machines. This inconsistency occurs no matter what you are using to create your website. When possible, use less-subtle tones and more contrasting colors to control for that difference.