The bug
Teams don't have the correct color in tab playerlist and on the sidebar before reconnecting.
How to reproduce
Create a team
/team add teamRed
Give that team a color
/team option teamRed color red
Join that team
/team join teamRed @s
Press tab and see that your name is white
Then try reconnecting
You now have the correct color on the tab playerlist
Confirmed for 18w14b