Its a feature from the mod odin which tells the team which dragon should they kill during the split. Theres 2 teams: bers and arch and they target 1-1 dragon. The mod sends a chat message in party chat when the dragons start to spawn. Heres an example:
ARCH TEAM ——> PURPLE | BERS TEAM ——> ORANGE
This feature is highly useful and basically a requirement to have in pb running parties, someone getting lost and targetting the wrong deagon is very likely to make the split fail.