Add hierarchal support to the categories legend from The Events Calendar Category Colors WordPress plugin (as mentioned at https://github.com/afragen/the-events-calendar-category-colors/issues/83)

legend.php

<?php
namespace Fragen\Category_Colors;

use Tribe__Events__Main;

$teccc = Main::instance();

$show_hidden = false;
$options = get_option( 'teccc_options' );
if ( isset( $options['show_ignored_cats_legend'] ) ){
	$show_hidden = true;
}
?>
<div id="legend_box" class="teccc-legend">

<?php
$taxonomies = array('tribe_events_cat');
$args = array(
	'hide_empty' => false,
	'fields' => 'all',
	'parent' => 0,
	'hierarchical' => true,
	'child_of' => 0,
);
$terms = get_terms($taxonomies, $args);
?>
<?php
$ignored_term_slugs = array();
foreach ( $teccc->ignored_terms as $ignored_term ){
	array_push($ignored_term_slugs,$ignored_term[0]);
}
?>
	<ul id="legend" class="hierarchical">
	<?php foreach ( $terms as $term ): ?>
		<?php if(in_array($term->slug,$ignored_term_slugs)): ?>
			<?php if($show_hidden): ?>
			<li class="teccc-hidden-category"><a href="<?php echo get_term_link($term->term_id); ?>"><?php echo $term->name; ?></a><input type="hidden" value="<?php echo $term->slug; ?>" /></li>
			<?php endif; ?>
		<?php else: ?>
			<li id="category-<?php echo $term->term_id; ?>" class="tribe-events-category-<?php echo $term->slug; ?> tribe-events-category-<?php echo $term->term_id; ?>"><a href="<?php echo get_term_link($term->term_id); ?>"><?php echo $term->name; ?></a><input type="hidden" value="<?php echo $term->slug; ?>" />
			<?php
			$subterms = get_terms($taxonomies, array(
				'parent'	 => $term->term_id,
				'hide_empty' => true
			));
			?>
				<ul>
				<?php foreach ( $subterms as $subterm ): ?>
					<?php if(in_array($subterm->slug,$ignored_term_slugs)): ?>
						<?php if($show_hidden): ?>
						<li class="teccc-hidden-category"><a href="<?php echo get_term_link($subterm->term_id); ?>"><?php echo $subterm->name; ?></a><input type="hidden" value="<?php echo $subterm->slug; ?>" /></li>
						<?php endif; ?>
					<?php else: ?>
						<li id="category-<?php echo $subterm->term_id; ?>" class="tribe-events-category-<?php echo $subterm->slug; ?> tribe-events-category-<?php echo $subterm->term_id; ?>"><a href="<?php echo get_term_link($subterm->term_id); ?>"><?php echo $subterm->name; ?></a><input type="hidden" value="<?php echo $subterm->slug; ?>" /></li><!-- #category-<?php echo $subterm->term_id; ?> -->
					<?php endif; ?>
				<?php endforeach; ?>
				</ul>
			</li><!-- #category-<?php echo $term->term_id; ?> -->
		<?php endif; ?>
	<?php endforeach; ?>
	</ul><!-- #legend -->

</div><!-- #legend_box -->

style.css

#tribe-events #legend.hierarchical { margin:1em 0; padding:0; }
#tribe-events #legend.hierarchical > li { display:block; float:none; text-align:left; }
#tribe-events #legend.hierarchical ul { margin:1em 0 0; padding:0; list-style:none; }
#tribe-events #legend.hierarchical li { margin:0.1em 0; }