How to get random teams from a function?

function getTeams()
    local teamNum = 1
    repeat wait()       
        local spectators = {}
        local teams = {"Red", "Blue"}
        local players = game.Players:GetChildren()
        for index, player in pairs(players) do
            if (player.TeamColor == game.Teams["Spectators"].TeamColor) then
                if (player.Character:findFirstChild("Torso")) then
                    table.insert(spectators, player)
                end
            end
        end
        local randomPlayer = spectators[math.random(1, #spectators)]
        randomPlayer.TeamColor = game.Teams[teams[teamNum]].TeamColor
        local spectatorNum = 0
        local players = game.Players:GetChildren()
        for index, player in pairs(players) do
            if (player.TeamColor == game.Teams["Spectators"].TeamColor) then
                if (player.Character:findFirstChild("Torso")) then
                    spectatorNum = spectatorNum + 1
                end
            end
        end
        teamNum = (teamNum == 1) and 2 or 1
    until (spectatorNum == 0)
end

What I have here is a function which gives a player a random team. The problem is that they aren't random. Every single person who joins is put onto red team. The point of this script being so long and complex is to make it so if someone joins halfway through a game, they don't cause everyone else to have their teams changed.