استفاده از طیف رنگ در CSS

#grad {

  background: -webkit-linear-gradient(red, blue); /* For Safari 5.1 to 6.0 */

  background: -o-linear-gradient(red, blue); /* For Opera 11.1 to 12.0 */

  background: -moz-linear-gradient(red, blue); /* For Firefox 3.6 to 15 */

  background: linear-gradient(red, blue); /* Standard syntax */

}

منبع:

http://www.w3schools.com/css/css3_gradients.asp