The premise of this is pretty simply to understand:
- For games like FFA / Assassinations, everyone is a neutral colour (yellow) so that there isn't an element of focusing that is involved. In the pre-lobby of Skywars, you can see the ranks there, but once you spawn in, it's changed, again, to stop focusing.
- For a game with colours (Line Dash, Wing Rush, EggWars ect), the colours of the team are used for consistency and to avoid confusion. If you were in a black team, and you are gold, making your name " [ B ] Younisco", a player can easily mistaken you for a yellow team player.
However, as you do, you may want to flaunt your rank -- you can just talk in chat in pre-lobbies and in game (solo games only, as it only shows up there). Those actions do have consequences though -- focusing may be more apparent. I hope this answers your suggestion. :)