From my understanding the whole thing started with a group of DISers who wanted to develop a way to recognize other DISers when in the parks.
They came up with the idea of using a color - whether it be your shirt, hair ribbon, etc. that might be a bit more unusual and would stand out - hence the Lime Green.
From there it kind of expanded and someone found the Mickey paint chips at the Home Depot and the "Alien Green" color is a shade of lime green.
So, we all liberate Alien/Lime Green Mickey Heads and use them in all sorts of ways to recognize other DISers in the parks:
Name Tags on Luggage/purse/backpack/stroller
Door & window decorations
Some DISers print out their DIS name on them and hand them out like business cards or calling cards when they run in to people or notice decorated doors and windows that indicate the DIS
Granted, I haven't even been down there yet, but this is just the information that I've gathered from being around here and have been liberating LGMHs like a good little DISer!