pewc_swatch_wrapper_classes.php

<?php
/**
 * Add custom class parameter to swatch options
 */
function demo_custom_class_param( $option_count, $group_id, $item_key, $item, $key ) {
	$name = '_product_extra_groups_' . esc_attr( $group_id ) . '_' . esc_attr( $item_key ) . '[field_options][' . esc_attr( $option_count ) . ']';
	$class = isset( $item['field_options'][esc_attr( $key )]['class'] ) ? $item['field_options'][esc_attr( $key )]['class'] : '';
	?>
	<td class="pewc-option-extra">
		<input type="text" class="pewc-field-option-class" name="<?php echo $name; ?>[class]" value="<?php echo esc_attr( $class ); ?>">
	</td>
<?php }
add_action( 'pewc_after_option_params', 'demo_custom_class_param', 20, 5 );

function demo_add_custom_class_title( $group_id, $item_key, $item ) {
	printf(
		'<th class="pewc-option-extra-title"><div class="pewc-label">%s</div></th>',
		__( 'Class', 'pewc' )
	);
}
add_action( 'pewc_after_option_params_titles', 'demo_add_custom_class_title', 20, 3 );

function demo_swatch_wrapper_custom_class( $wrapper_classes, $item, $key, $option_value, $option_index ) {
	if( ! empty( $option_value['class'] ) ) {
		$wrapper_classes[] = esc_attr( $option_value['class'] );
	}
	return $wrapper_classes;
}
add_filter( 'pewc_swatch_wrapper_classes', 'demo_swatch_wrapper_custom_class', 10, 5 );