Just did another mod to my Garmin system. I stopped at the parts house and bought some shrink tubing. One color for each collar.
In the past I have always labeled my collars by number, that I etched into the housing. Sometimes with mud and dirt buildup, the etching was hard to read. Now I will have an easy to identify color antenna on each collar.

Now THAT is an awesome idea! Everything that I have tried to mark mine with has either come off or become unreadable.... Including etched numbers. I'm going to have to try that.
Steve