Local script
local player = game.Players.LocalPlayer
script.Parent.MouseButton1Click:Connect(function()
player.TeamColor = BrickColor.new("Bright green")
player.Character.Humanoid.Health = nil
script.RemoteEvent:FireServer()
end)
script
script.Parent.OnServerEvent:Connect(function(player, TeamColor, Character)
player.TeamColor = TeamColor
end)

4 comments
You forgot to put the variables in ":FireServer()", the function doesn't know who is the player, neither the TeamColor. You should put ":FireServer(player, player.TeamColor)".
Not completely true maquilaque, 'player' will be send automaticly client - server
So when I call the player it is called twice? so is there any point of calling the player?
If you go client -> server; you don't have to add player, this changes for server -> client (as you need to specify the player), but not for :FireAllClients (as it fires for all players)