var swatches = YAHOO.util.Dom.getElementsByClassName('swatch');
YAHOO.util.Event.addListener( swatches, 'mouseover', hoverColor );

function hoverColor( e ) {
   var imgID = this.name.replace('-'+this.id, '');
   var img = 'img/category/'+this.id+'a-med-cat.jpg';
   document.getElementById(imgID).src = img;
}