DataStore save only one team?

First of all, you should be declaring your datastore variable outside of the functions. This will save memory, as you won't be creating a new one each time the function runs. Additionally, :connect() is deprecated. Use :Connect() instead. Now onto the big issue.

:SetAsync() requires two arguments: key, and value, respectively. You have the value (the player's TeamColor), but you're missing the key. When saving individual player's values, you should be using a unique key. This is typically the player's UserId, sometimes accompanied by a string value.

local DSS = game:GetService("DataStoreService")
local DataStore = DSS:GetDataStore("PlayerTeamColor")

local DefaultTeamColor = BrickColor.new("Really red") --change this to what the default should be if no data has been previously saved.

game.Players.PlayerAdded:Connect(function(player)
   local key = "player_" .. player.UserId
   player.TeamColor = datastore:GetAsync(key) or DefaultTeamColor
end)

game.Players.PlayerRemoving:Connect(function(player)
   local key = "player_" .. player.UserId
   if player.TeamColor == BrickColor.new("Medium stone grey") then
      datastore:SetAsync(key,player.TeamColor)
   end
end)

Notice how I assigned a default value to the player's TeamColor if :GetAsync() returns nil. If we don't do this, the script will error if the returned data is nil.

##Resources:

:SetAsync()

##Accept and upvote if this helps!