I found a grey/silver hair amongst my eyelashes. (I can almost imagine God/Jesus/Buddha/Allah/Yahweh...laughing at me and saying "Let's see you pluck that one out.)
When I see them in my hair, I pluck them out. I get reddish highlights put in my hair every now and again, but I don't go beyond that. I'll only continue do anything with color up until the point where it looks ridiculous. (I won't be sitting around at 90 with dark hair, unless I continue to be well-preserved in other words.)
I haven't seen any in my eyebrows.