how to make countdown timer changes text color after reaching last 10 seconds?

If this script is from the server, fire a remote function from the script and receive it on the client in a Local Script (usually in the GUI).

Example:

-- Server Script
for i = GameLength, 0, -1 do
	local minutes = math.floor(i/60)
	local seconds = math.floor(i%60)
	Status.Value = (string.format("%i:%.2i", minutes, seconds))
	wait(1)

	-- Check if there is 10 seconds left
	if Status.Value == "00:10" then
		for _, player in pairs(game.Players:GetPlayers()) do
			game.ReplicatedStorage.ChangeTextColor:InvokeClient(player)
		end
	end
end

-- Local Script
game.ReplicatedStorage.ChangeTextColor.OnClientInvoke = function()
	script.Parent.Parent.YourGui.TextLabel.TextColor3 = Color3.fromRGB(255,0,0)
end

I hope this helped!