With logging

output = 0
local h = fs.open('log', 'a')

-- just a formatting line
h.write('----------------------------\nDay: '..os.day..' Time: '..textutils.formatTime(os.time())..' Clock: '..os.clock()..'\n----------------------------')

rednet.open('left')

local function log(msg)
  -- write the error to a file
  h.write(msg..'\n')
  h.flush()

  -- print it to the screen
  print(msg)

  -- if using cc-emu and you have the latest version print it to the log
  if emulog then emulog.log(msg) end
end


while true do
  -- get our message
  local sender, msg, dist = rednet.receive()
  if sender == 45 and #msg == 2 then

    -- get the first part of our message and convert it from hex (base16) to decimal (base10)
    local num = tonumber(msg:sub(1,1), 16)

    -- if it was not hexadecimal
    if not num then
      log('Invalid message (num not hex) >>> sender: '..sender..' msg: '..msg)
    else -- if it was hex
      -- decide which function we should use based on if the second character is a t (for true)
      action = (msg:sub(2,2) == 't') and colors.combine or colors.subtract

      -- call out action with the current output and the one we just got
      output = action(output, 2 ^ num)

      -- everything is valid now lets output
      rs.setBundledOutput('back', output)
    end
  else
    log('Invalid message (invalid sender) >>> sender: '..sender..' msg: '..msg)
  end
end

rednet.close('left')