Why are some users greyed out?

If you have the data explorer plugin, there’s a pre-made report in there for ‘Inactive Users with no posts’. You could also tweak it a little to give you an interactive list where you can click through to their admin page:

SELECT 
    u.id AS user_id, 
    '/admin/users/' || u.id || '/' || u.username_lower AS admin_page_url, 
    u.created_at, 
    u.last_seen_at
FROM users u
WHERE u.active = false
ORDER BY u.id DESC