✓ Accepted answer
First problem
player.TeamColor takes a BrickColor value not a string
BrickColor.new("Bright blue")
Second problem
the parameters of MouseButton1Down are x, y not the player that clicks it.
MouseButton1Down should be used in a local script since so you can get the person who clicked with localplayer
local clicker = game.Players.LocalPlayer
Third problem
Humanoid isn't in the player, it's in the character
local clicker = game.Players.LocalPlayer
local character = clicker.Character
Fixed code put together
local clicker = game.Players.LocalPlayer
local character = clicker.Character
script.Parent.MouseButton1Down:connect(function()
if clicker.TeamColor == BrickColor.new("Bright blue") then
clicker.TeamColor = BrickColor.new("Bright red")
clicker.Humanoid.Health = 0
end
end)