There are plenty of "old wives tales" around how to treat the common problem of stained skin as a result from dyeing hair. If you can act quickly, your best bet is to lightly wet a cotton ball with some warm water and gently wipe the "slipped" hue away before it can set into the skin (it usually takes several minutes before the color has a chance to truly permeate).
The other foolproof method lies in preventing hair color skin stains before they start. Before you put on your colorist gloves, apply a dab of conditioner, Vaseline or lip balm around the hairline and other areas you know are subject to dripping color (like earlobes, eyebrows, the jawline and the nape of your neck). Adding moisture to sit on the skin will provide a "barrier" that color can't attach to or stain. Further, having a visual cue of your "danger zones" can help to mentally manage your color application.
Removing hair color stains
There are loads of other "old wives tales" out there when it comes to removing hair color from the skin. Though many of these will depend on how much color has seeped, how permanent the blend is and where it is located, one of these "home-tested" methods might just do the trick for you: