I'm in the process of building a paint ball sentry gun to use this summer. The plans can be found here: http://projectsentrygun.rudolphlabs.com/ I have ran into one problem though the camera for the gun picks up all movement and shoots it, even me. Is there some kind of sensor I can ware that can communicate with another sensor on the arduino when its in the cameras field of view that I can program so it doesn't shoot me. I can figure out the code my self I just don't know what sensor I need.
Reminds me of this XKCD cartoon.
Based on the website isn't that what "safe color" is for?
Otherwise you may consider something like bluetooth- there's lots of projects on the internet around recognizing a particular phone by its bluetooth address and opening a door or turning on a light. The HC-06 module is cheap and well supported for arduino applications