Need help in indexing a part inside a model that has multiple names [randomly generated]?

room = script.Parent.Parent.Parent

while wait(0.1) do
    if room.Human:FindFirstChildOfClass("Model") then
    script.Parent.Humanoid.LeftArm.BrickColor = BrickColor.new("Crimson")
    print("Arm")

    else
    script.Parent.Humanoid.LeftArm.BrickColor = BrickColor.new("Medium stone grey")
    print("No arm")

    end
end

In this state the script works but after changing this:

if room.Human:FindFirstChildOfClass("Model") then

to this:

if room.Human:FindFirstChildOfClass("Model").Part["Left Arm"] then

it stops working and gives an error [indexing nil value]. How can I make it work? I tried multiple indexing ways but none worked.

I want to index a body part of a humanoid model that has randomly generated name. Script checks if the humanoid has body parts and colours them [red] on a hud if they are present and [grey] if they are gone.