CSS3 предоставляет способ сделать это
background-image: linear-gradient(bottom, #FFD51A 50%, #FAC815 50%);
background-image: -o-linear-gradient(bottom, #FFD51A 50%, #FAC815 50%);
background-image: -moz-linear-gradient(bottom, #FFD51A 50%, #FAC815 50%);
background-image: -webkit-linear-gradient(bottom, #FFD51A 50%, #FAC815 50%);
background-image: -ms-linear-gradient(bottom, #FFD51A 50%, #FAC815 50%);
http://jsfiddle.net/WnwNW/1/