What is best brow pencil colour for mostly grey hair. Original hair light brown and now mostly grey.

I would recommend you think of grey hair as a cool light hair colour and then to determine the tone of your own brows we must consider skin shade as always. For example for a natural look if your skin is dark or quite tan with unbleached brow hair you wouldn't want to use a blonde pencil regardless the hair colour anyway. Medium Blonde or Dark Ash are good places to start. Good luck!