INSERT INTO wp_usermeta
(
user_id,
meta_key,
meta_value
)
(
SELECT
id,
'admin_color',
'classic'
FROM wp_users
WHERE id  NOT  IN (SELECT user_Id FROM wp_usermeta WHERE meta_key = 'admin_color')
)
 
UPDATE wp_usermeta SET meta_value = 'classic' WHERE meta_key = 'admin_color'