filter theme.json

function filter_theme_json_theme( $theme_json ){
	$new_data = array(
		'version'  => 2,
		'settings' => array(
			'color' => array(
				'text'       => false,
				'palette'    => array( /* New palette */
					array(
						'slug'  => 'foreground',
						'color' => 'black',
						'name'  => __( 'Foreground', 'theme-domain' ),
					),
					array(
						'slug'  => 'background',
						'color' => 'white',
						'name'  => __( 'Background', 'theme-domain' ),
					),
				),
			),
		),
	);

	return $theme_json->update_with( $new_data );
}
add_filter( 'theme_json_theme', 'filter_theme_json_theme' );

do_action( ‘wp_after_load_template’, $_template_file, $require_once, $args )