How do I add Fire and BodyColor Changing to this Script?

You would have to make a variable making the fire, sparkles, or smoke. This is how you would do it.

particles = Instance.("Fire",hit.Parent.Torso)

The full script would be:

script.Parent.Touched:connect(function(hit)
    if hit and hit.Parent and hit.Parent:FindFirstChild("Humanoid") then
        hit.Parent.Humanoid.Health = 0
    particles = Instance.new("Fire",hit.Parent.Torso)
    end
end)

You can replace fire with Sparkles, Smoke, and ParticleEmitter

(This is in addition to docrobloxman52's answer)

To turn all of the character's parts black, you can iterate over the character's parts:

parts = hit.Parent:GetChildren()
for i = 1, #parts do
    if parts[i]:IsA("BasePart") then
        parts[i].BrickColor = BrickColor.new("Black")
    end
end

(Regarding your comment) If you wish to access a specific part with a space in the name, you can use :FindFirstChild to make sure it's there, or simply use hit.Parent["Left Arm"] (but that will error if Left Arm doesn't exist). So, if you wanted to change their Left Arm to Red, you might do:

local leftArm = hit.Parent:FindFirstChild("Left Arm")
if leftArm then
    leftArm.BrickColor = BrickColor.new("Bright red")
end