I just found out about this myself, so I hope I don't get anything wrong.
I guess at Home Depot they have Disney paint, and this "alien" green color comes with a Mickey-shaped head.
DIS members write their user names on them and put them on their hotel doors, book bags, etc. so that everyone can recognize them in the parks.
Just a neat way to connect in the parks. I haven't been able to do it yet myself, but on my next trip I will.
I'm sure someone will fill in the blanks of what I may have missed.