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

colors = {}

for y=0, h-1 do
  colors[img.getInt(0, y)] = 1
  colors[img.getInt(w-1, y)] = 1
end

for x=0, w-1 do
  colors[img.getInt(x, 0)] = 1
  colors[img.getInt(x, h-1)] = 1
end

list, n = {}, 0
for color, _ in pairs(colors) do
  n=n+1
  list[n] = color
end

if #list > 1 then
  return "multi-color border"
else
  return "border color: "..colortostring(list[1])
end