You can not address an element that is referenced via use. The specs say:
For user agents that support Styling with CSS, the conceptual deep cloning of the referenced element into a non-exposed DOM tree also copies any property values resulting from the CSS cascade ([CSS2], chapter 6) on the referenced element and its contents. CSS2 selectors can be applied to the original (i.e., referenced) elements because they are part of the formal document structure. CSS2 selectors cannot be applied to the (conceptually) cloned DOM tree because its contents are not part of the formal document structure.
Nevertheless, Firefox supports addressing “virtual” elements the are included via a use wormhole. All other browsers don’t.
What more browser do support is changing filling or stroking color if you give the referenced element a fill/stroke value of currentColor and then you change the color property of the <use> element on hover. Like so:
<svg version="1.1" width="640" height="480" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<style type="text/css">
#p0 {fill:currentColor}
#use1:hover {color:green}
#use2:hover {color:red}
#use3:hover {color:blue}
</style>
<defs>
<polygon id="p0" points="100,0 50,86.6 -50,86.6 -100,0 -50,-86.6 50,-86.6" class="active" />
</defs>
<g transform="translate(70,100)">
<use xlink:href="#p0" transform="translate(40,0)" id="use1" />
<use xlink:href="#p0" transform="translate(250,0)" id="use2" />
<use xlink:href="#p0" transform="translate(460,0)" id="use3" />
</g>
</svg>This is supported by all major browsers (FF, Chrome, IE, Safari). Only Opera doesn’t seem to like it. The disadvantage is of course, with this method you can only change one color.
So, a different method would possible be to use filters, if it’s only about changing color. For example using <feColorMatrix>, you can transform one color to another using a color matrix, like this:
<svg version="1.1" width="640" height="480" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<style type="text/css">
#p0 {fill: currentColor}
#use1:hover {filter: url(#filter1)}
#use2:hover {filter: url(#filter2)}
#use3:hover {filter: url(#filter3)}
</style>
<defs>
<g id="p0">
<polygon points="100,0 50,86.6 -50,86.6 -100,0 -50,-86.6 50,-86.6" fill="red" />
<rect width="50" height="70" fill="green" />
<circle cx="-20" cy="-30" r="30" fill="blue" />
</g>
</defs>
<filter id="filter1">
<feColorMatrix type="matrix" in="SourceGraphic" values="0 1 0 0 0
1 0 0 0 0
0 0 1 0 0
0 0 0 1 0" />
</filter>
<filter id="filter2">
<feColorMatrix type="matrix" in="SourceGraphic" values="0 0 1 0 0
1 0 0 0 0
0 1 0 0 0
0 0 0 1 0" />
</filter>
<filter id="filter3">
<feColorMatrix type="matrix" in="SourceGraphic" values="0 1 0 0 0
0 0 1 0 0
1 0 0 0 0
0 0 0 1 0" />
</filter>
<g transform="translate(70,100)">
<use xlink:href="#p0" transform="translate(40,0)" id="use1" />
<use xlink:href="#p0" transform="translate(250,0)" id="use2" />
<use xlink:href="#p0" transform="translate(460,0)" id="use3" />
</g>
</svg>Still no luck with Opera, though, and this time I wasn’t happy with IE9 and Safari either. But I believe it should be possible with Opera and Safari, only I did something not 100% correctly.