Echo ACF custom field in body class

acf_body_class.php

// ACF field added to body class
// Adapted from http://krogsgard.com/2012/wordpress-body-class-post-meta/

add_filter('body_class','tend_custom_field_body_class');

function tend_custom_field_body_class( $classes ) {
    global $wp_query;
    $postid = $wp_query->post->ID;
    
	if ( is_page_template('template-service-index.php')  ) {
		$classes[] = 'variant-'.get_field('colour_scheme', $postid);
	}
	// return the $classes array
	return $classes;
}