Before talking about “how to adjust skin tone” I think it’s worth spending some words on the calibration of the monitor on OS X.
Have you ever experienced to see your website or photo on a pc monitor and to say “Hey, on my Mac the colours were of a higher quality!”?
Well, you have to consider the fact that now most computers have adopted the o.s. Windows; so, as a consequence, you have to get accustomed and create a personalized profile colour, just to avoid problems of visualization and accept this, ehm, “tragedy”.

This is a self-made solution, obviously, if you can spend some money more, you can buy a device able to calibrate your monitor, for example EyeOne LT.

If you decide to do this, just go to system preferences > monitor > color and choose to calibrate your monitor.
System preferenceMonitorCalibrate
Select export mode and go on till the gamma setting.

Check expert modeChoose gamma mode 2.2

Now choose 2.2, then flag the option use native white point, as all Apple monitors are already set on a white point D65 (about 6500°K).
Now you can save this configuration then click continue to exit.

Check expert modeCheck expert mode

In general, on the macs the pictures tend to be brighter than on the computers: this mainly because of a different setting of the gamma and the white point.

You can try SuperCal, a 19$ shareware, in alternative to Color sync.

I’ve found this usefull info on Apple support website, click here if you want to read more.