Yes – because the client is changing that property without communication with the server, the server script will not know that the player’s TeamColor changed, nor will anyone else in the server.
In order to resolve this, you need to be changing the TeamColor property from a server script – if this is for a Team-Change GUI, you can communicate between the Client and the Server using RemoteEvents and have the client let the server know when the player needs to be placed onto a new team.