It sounds like the issue is (yet again) monitor calibration. If you're using the same color profile (sRGB, etc.) the values will be the same regardless of the OS used.
You and your designer should agree on a color profile (there are many, many profiles and they're mostly OS independent) simply for the sake of consistency. Since color is part of design I'd let them take the lead in that part. Color profile can't really be set in the OS; it really comes into play with browsers and print materials.
If you're concerned about the display not matching you could always calibrate your monitor with a calibration tool. Unfortunately, even if your display matches that of your designer's it's not going to match what's out there in the real world - no one outside of design studios really calibrates their monitors so despite your hard work it's always going to look different on end-users' displays.