what version of Excel are you using? I have office 2000 pro and don’t find the setting that I have found in another post but a user of OS X using office X says that if you go into General Preferences under System Preferences there is a Highlight Color setting. They say once you change it you have to close and reopen Excel to have the changes show up. I am still looking for this setting in office 2000 for windows.
update – one user has told me that you have to go into your display properties. Start > Settings > Control Panel > Display > Appearance > Advanced. Then choose the SELECTION option and change the color there. This does work for me, but it will change your selection color for all apps, not just excel.