code: function findClosest(c) {
  
      if (colorTab === null) return -1;
      var r = (c & 0xFF0000) >> 16;
      var g = (c & 0x00FF00) >> 8;
      var b = (c & 0x0000FF);
      var minpos = 0;
      var dmin = 256 * 256 * 256;
      var len = colorTab.length;
  
      for (var i = 0; i < len;) {
        var dr = r - (colorTab[i++] & 0xff);
        var dg = g - (colorTab[i++] & 0xff);
        var db = b - (colorTab[i] & 0xff);
        var d = dr * dr + dg * dg + db * db;
        var index = i / 3;
        if (usedEntry[index] && (d < dmin)) {
          dmin = d;
          minpos = index;
        }
        i++;
      }
      return minpos;
    }
docs: Returns index of palette color closest to c