Regenerate Health Gui not working?

Hello, please make sure to indent your code, you also should create variables as they make things a lot easier and really prevents some issues from happening with your code, It seems also that the code:

if game.Players.LocalPlayer.Character.Humanoid.Health > yellowzone then
    local brickcolor = BrickColor.new("Lime green")
    local color = brickcolor.Color
    script.Parent.BackgroundColor3 = color
end

is not needed as if Humanoid.Health is already higher than the redzone It'll already change to Lime green.

I've modified your code in a way that it works with the GUI I've tested which should work for yours too, Once again I suggest to make variables as they're not evil and they're also really helpful!

In order to regenerate the player's health you'll need a remoteevent since you cannot do it from a localscript, in this case I created a remoteevent called " RegenHealth " in workspace which will be used to regenerate the player's health.

Modified localscript:

-- Try to put variables as they're really helpful!
local LocalPlayer = game:GetService("Players").LocalPlayer
local Character = LocalPlayer.Character or LocalPlayer.CharacterAdded:wait()
local Humanoid = Character:WaitForChild("Humanoid")
local RegenEvent = game.workspace.RegenHealth

Humanoid.Changed:Connect(function()

    local redzone = (Humanoid.MaxHealth / 4)
    local yellowzone = (Humanoid.MaxHealth /2)

    if Humanoid.Health <= redzone then -- If Humanoid.Health is lower or equal than redzone
        script.Parent.BackgroundColor3 = Color3.new(255,0,0)
    end

    if Humanoid.Health > redzone then -- If Humanoid.Health is higher than redzone
        local color = BrickColor.new("Lime green").Color
        script.Parent.BackgroundColor3 = color
    end

    script.Parent.Size = UDim2.new(Humanoid.Health / Humanoid.MaxHealth,0,0,20) -- Changes the size of the gui with the Health and MaxHealth the player currently has.
end)

while wait(1) do -- Change 1 to the seconds you want it to regenerate
    RegenEvent:FireServer(1) -- Change the 1 to how much you want it to regenerate
end

ServerScript ( Inside remoteevent )

script.Parent.OnServerEvent:Connect(function(Player, HealthAmount)
    local Character = Player.Character or Player.CharacterAdded:Wait()
    local Humanoid = Character:WaitForChild("Humanoid")

    Humanoid.Health = Humanoid.Health + HealthAmount
end)

Hopefully this solves your issue! Make sure to accept the answer if it did.