What's supposed to happen is adjacent repeaters on the same frequency should have different color codes, so that a transmission from a subscriber unit does not activate an unintended co-channel repeater.
Color codes come from colors in a map, where you want countries that touch to have different colors. The four color theorem says you can color any map with 4 different colors. If you want to be separated by two neighbors, you need 7.
PL tones act as color codes, and digital color codes were used as far back as AMPS analog cellular (on the control channels). 802.11ax Wi-Fi now has color codes.