create a linearGradient to fill the star. The 2 stops ensure an instant transition.
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="256px" height="256px" viewBox="0 0 32 32">
<defs>
<linearGradient id="grad">
<stop offset="50%" stop-color="yellow"/>
<stop offset="50%" stop-color="grey"/>
</linearGradient>
</defs>
<path fill="url(#grad)" d="M20.388,10.918L32,12.118l-8.735,7.749L25.914,31.4l-9.893-6.088L6.127,31.4l2.695-11.533L0,12.118
l11.547-1.2L16.026,0.6L20.388,10.918z"/>
</svg>
then set the fill of the star to this.