Bananavice

This is a bit late, but this is exactly the same thing that I needed to do. I tried for a while to push the class into the widget options in different ways but failed with everything so I just went with a preg_replace on $before_widget.

This is how it looked for me:

function widget($args, $instance) {
	extract($args);

	$color = $instance['color'];
	$before_widget = preg_replace('/class="(.*?)"/i', 'class="$1 '.$color.'"', $before_widget);

	echo $before_widget;
		echo '<h3>'.$instance['title'].'</h3>';
		echo '<div class="text">'.$instance['text'].'</div>';
	echo $after_widget;
}