function invertHex(hexnum){
  if(hexnum.length != 6) {
    alert("Hex color must be six hex numbers in length.");
    return false;
  }
	
  hexnum = hexnum.toUpperCase();
  var splitnum = hexnum.split("");
  var resultnum = "";
  var simplenum = "FEDCBA9876".split("");
  var complexnum = new Array();
  complexnum.A = "5";
  complexnum.B = "4";
  complexnum.C = "3";
  complexnum.D = "2";
  complexnum.E = "1";
  complexnum.F = "0";
	
  for(i=0; i<6; i++){
    if(!isNaN(splitnum[i])) {
      resultnum += simplenum[splitnum[i]]; 
    } else if(complexnum[splitnum[i]]){
      resultnum += complexnum[splitnum[i]]; 
    } else {
      alert("Hex colors must only include hex numbers 0-9, and A-F");
      return false;
    }
  }
	
  return resultnum;
}

http://www.mattlag.com/scripting/hexcolorinverter.php

By Matt Lagrandeur

Hexadecimal Color Inverter

css, web, color

JavaScript