var hexDigits = new Array
("0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f");
//Function to convert rgb color to hex format
function rgb2hex(rgb) {
rgb = rgb.match(/^rgb\((\d+),\s*(\d+),\s*(\d+)\)$/);
return "#" + hex(rgb[1]) + hex(rgb[2]) + hex(rgb[3]);
}
function hex(x) {
return isNaN(x) ? "00" : hexDigits[(x - x % 16) / 16] + hexDigits[x % 16];
}
(Source)