Decal Part When Touched?

This is the basic knowledge for beginners. I won't go through the bulk explanations, learn it through the codes.

local Part1 --Your part1
local Part2 --Your part2
local DECAL_ID = "rbxassetid://ID_HERE"--Type Id here, you know how.
local DECAL_FACING = Enum.NormalId.Front--[[Decal facing direction.((Related Vector3))--]]
local DECAL_TRANSPARENCY = 0;--[[1 = Invisible, 0 = Visible--]]
--[[There are two methods:Part1 touches Part2 or Part2 touches Part1
    My method is Part2 touches Part1.
-]]
Part2.Touched:Connect(function(part)
    if part and part == Part1 and not part:FindFirstChildWhichIsA("Decal") then
        print("Part2 touched Part1, Part1 has no decal, adding decal!")
        local decal = Instance.new("Decal",Part1)
        decal.Face = DECAL_FACING
        decal.Texture = DECAL_ID
        decal.Transparency = DECAL_TRANSPARENCY
    end
end)

After this, we can convert it into a function:

-----------ESTABLISHING EVENT FUNCTIONS
local Established = {}
function Establish_TouchEvent(p1,p2,DECAL_ID,DECAL_FACING,DECAL_TRANSPARENCY)
    --We use p1 as a key, p2 as a value.
    --Which won't cause any overlapping problems
    if not (Established[p1] == p2 or p1 == nil or p2 == nil) then
        Established[p1] = p2
        p2.Touched:Connect(function(part)
            if part and part == p1 and not part:FindFirstChildWhichIsA("Decal") then
                local decal = Instance.new("Decal",p1)
                decal.Face = DECAL_FACING
                decal.Texture = DECAL_ID
                decal.Transparency = DECAL_TRANSPARENCY
            end
        end)
    end
end

Use it like this: Establish_TouchEvent(p1,p2,"your decal id",Enum.NormalId you know,Transparency)

More details: BasePart.Touched