I want to change a person's Skin color when I press "E" and it does but its PREMANENT!?

Save the skin color for each of the players parts in a temporary table when they press E the first time then make a debounce so for example:

local EPressed = false

if EPressed == false then
    --Change their color.
elseif EPressed == true then
    --Turn their color back
end

After the first time it will log their skin color for their parts in a temp table and when they press E the second time it grabs from that table for each part and changes it to the color logged.

Example

local EPressed = false
local player = game.Players.LocalPlayer
local colorTable = {}

if EPressed == false then
    EPressed = true
    for i,v in pairs(player.Character:GetChildren()) do
        if v:IsA("BasePart") or v:IsA("MeshPart") then
            table.Insert(colorTable, v.BrickColor)
            v.BrickColor = BrickColor.new("Terra Cotta")
        end
    end
elseif EPressed == true then
    EPressed = false
    for i,v in pairs(player.Character:GetChildren()) do
        if v:IsA("BasePart") or v:IsA("MeshPart") then
            v.BrickColor = BrickColor.new(colorTable[i])
        end
    end
end

You could loop through the parts and save their BrickColor. And adding on to LegitimateTrade's answer, you could have a boolean to check if it was pressed.

local Players = game:GetService("Players")
local player = Players.LocalPlayer -- to prevent having to type out the full thing
local character = player.Character or plr.CharacterAdded:Wait()
local enabled = false -- We will set to true when button is pressed
local tc = "Terra Cotta" -- brick colour
local bodyColours = {} -- blank table that we will add body colours to.

uis.InputBegan:Connect(function(input, gpe)
    if gpe then return end -- stop if they're chatting

    if input.KeyCode == Enum.KeyCode.E then
        enabled = not enabled -- set to the opposite of current state

        for _, v in pairs(character:GetChildren()) do -- loop through children
            if v:IsA("BasePart") then -- in the char, only parts have brick colours 
                table.insert(bodyColours, v.BrickColor) -- save obj
            end 
        end

        if enabled then -- if it's enabled
            for _, v in pairs(character:GetChildren()) do -- another for loop
                if v:IsA("BasePart") then v.BrickColor = BrickColor.new(tc) end
           end

        else-- it's not enabled, reset their brick colours

            for _, v in pairs(character:GetChildren()) do  
                if v:IsA("BasePart") then
                    for _, colour in pairs(bodyColours) do
                        v.BrickColor = colour
                    end
                end
            end

        end
    end
end)