Scope correction

bg = display.newRect (100,100, 300, 300)

local countdown

local function listener (event)
    local r = math.random (1,255)
    bg:setFillColor (r,r,r)
    countdown = nil
end

local function tl (event)
    if event.phase == "ended" then
        if countdown then
            timer.cancel (countdown)
            countdown = nil
        end
        countdown = timer.performWithDelay(500, listener)
    end
end

bg:addEventListener ("touch", tl)