Psyonix announced that with the new upcoming free rumble update that brings a new game mode with randomized power-ups, there will be custom team names and colors for private matches and much more stuff.
According to Rocket League’s official website “When spinning up a Private Match with friends and teammates, you’ll soon be able to add custom Team Names as well. These names will appear on the scoreboard whenever a goal is scored, and all players in the match will be able to see them.”
Along with custom Team Names, Private Matches will also support Team Color selection. That means that teams will be assigned any of the normal Primary and Secondary colors and that Stadium hues will change based on your selections. One thing to note, however, is that for gameplay reasons, you won’t be able to change a car’s primary color to white or black — just secondaries. They’ll still look pretty cool, though!”