I am having trouble covering grey hair with wella 6/3, last time I did it I left if for 30 mins and my client said her grey was coming through again after only 10 days!? Next time I will leave on for 40 mins but I'm not sure if I should add a base or not, and not sure which colour I should add as the base?? I have only just finished training so any advise on how I can cover the grey would be much appreciated thanks