In order to set a default WordPress admin color scheme for new users, all you need to do is add the following code in your theme’s functions.php file or in a site-specific plugin:
function set_default_admin_color($user_id) {
$args = array(
'ID' => $user_id,
'admin_color' => 'blue'
);
wp_update_user( $args );
}
add_action('user_register', 'set_default_admin_color');