function col2hex(num)
  local hexstr = '0123456789abcdef'
  local s = ''
  for i=1, 6 do
    local mod = math.fmod(num, 16)
    s = string.sub(hexstr, mod+1, mod+1) .. s
    num = math.floor(num / 16)
  end
  return s
end
    
function colortostring(c)
  if c == 0xFFFFFF then return "white" end
  return col2hex(c)
end

w, h = img.width, img.height

ofs = 0

while ofs <= w/2 and ofs <= h/2 do
  colors = {}
  
  for y=ofs, h-ofs-1 do
    colors[img.getInt(ofs, y)] = 1
    colors[img.getInt(w-ofs-1, y)] = 1
  end
  
  for x=ofs, w-ofs-1 do
    colors[img.getInt(x, ofs)] = 1
    colors[img.getInt(x, h-ofs-1)] = 1
  end
  
  list, n = {}, 0
  for color, _ in pairs(colors) do
    n=n+1
    list[n] = color
  end
  
  if #list > 1 then
    return ofs == 0 and "no border found" or "border thickness: "..ofs
  elseif lastcolor == nil then
    lastcolor = list[1]
  elseif lastcolor ~= list[1] then
    return ofs == 0 and "no border found" or "border thickness: "..ofs
  end
  
  ofs = ofs+1
end

--return "unicolor "..colortostring(lastcolor)
return "unicolor "..lastcolor

Began life as a copy of #287