What is wrong with this health bar script?

The purpose is to have a health bar, a bit like Skyrim's where it gets closer to the middle, depending on health, but the second part will change its colour, i'm sure I went wrong in several places, none of it works, I could of gone wrong with (I have given the GUI a size and colour to begin with, in its actual properties). I'm fairly sure GUIs do not have brickcolors, but I don't know how to put the Red, Blue, Green properties in it.

It is meant to be unique for each player too. The script is in the Starter GUI, inside a Screen GUI, inside a Frame.

local Frame= script.Parent
local health = script.Parent.Parent.Parent.Parent.Character.humanoid.Health.Value


while true do
    wait(0.1)
    Frame.Size=Frame.Size.new{0, health},{0, 5}
end

while true do
    wait(0.05)
    if health==100>=75 then
    Frame.BackgroundColor3= Frame.BackgroundColor3.new ("Dark Green")
    end -- If your health is from 100 to 75 then make the bar, this color
    elseif 
    health==74>=50 then
    Frame.BackgroundColor3= Frame.BackgroundColor3.new ("Grime")
    end 
    elseif 
    health==49>=25 then
    Frame.BackgroundColor3= Frame.BackgroundColor3.new ("Cool Yellow")
    end 
    elseif 
    health==24>=0 then
    Frame.BackgroundColor3= Frame.BackgroundColor3.new ("Bright Red")
    end 
end