Hello!
I always ask the client how dark they would like their eyebrows.
For a client with grey hair I would ask that question aswell, however I would normally use a mixture between a light brown and then brown. I feel like a light brown doesn’t last aswell so with a little bit of a darker brown mixed into it it helps keep the longevity,
Sometimes aswell what I do is I have a light brown and keep it on for a minute , show the client and then gradually build up the colour, that way you don’t have it too harsh and then you know for next time what they’re looking for. Hope this helps xxx