Praise be to Allah.
Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) said:
The Sunnah is to change the grey hairs by using henna, or henna and katam. If it is changed using henna it will be orange and if it is changed using henna and katam it will be between red and black. There is nothing wrong with that and no sin involved, in sha Allah.