How could I make a Brick perform the Function??

So I am making a game where you can Sword Fight and get Player Points for each kill. I don't want there to be a map. I am trying to make it to where you step on a certain plate and then you can kill each other. I came here to ask how I would go about making a brick to where you cannot kill each other with swords when your on it. The reason I have not made one yet is because I don't want that ugly FF around them I just want them to immune to getting damaged while on this brick. Also I had made one to where it gives the unlimited health but the bar changed to yellow like when you have GOD mode on but I don't like that I want it to stay green. This is not a request you do not have to help neither did I ask for you to make anything so please don't claim that it is! Have a good day!! ^_^