ideally, i would like to know if there exists an led / switch type of device so that when you touch it with a special pen it changes state, ie color ?
I've never heard of such a thing but that doesn't mean much. I'd do something like that with a simple reed switch and a magnet in the tip of the pen. If you go for a normally open reed switch you then wire it as a digital input to your arduino and then monitor it for a state change. Bring the pen near it and it will close the switch, activating your state change and then you use that to change your programming code (eg turn on then off, cycle through a set of states etc).
From memory there's an example sketch in the IDE that does something like this with a momentary switch that would work the same way.
Cheers
ajfisher
You could also do the same thing with a hall effect sensor and not have the click or magnetic "pull" from the reed switch.