javascriptはrgbの色を16進数に変換することを実現します.

function zero_fill_hex(num, digits) {
 var s = num.toString(16);
 while (s.length < digits)
  s = "0" + s;
 return s;
}
function rgb2hex(rgb) {

 if (rgb.charAt(0) == '#')
  return rgb;
 
 var ds = rgb.split(/\D+/);
 var decimal = Number(ds[1]) * 65536 + Number(ds[2]) * 256 + Number(ds[3]);
 return "#" + zero_fill_hex(decimal, 6);
}