I must admit, that I too am in this position. I cannot remember my hair being its natural colour (dull mousy brown) as it was highlighted in my 30s and has been red during my 40s.
My kind OH hennas it about every 6 weeks with Lush and I must admit, it takes years off my age and I get a lot of complements on the colour, but whenever I see the grey parting growing though, I wonder about going "natural". (Even writing this I am talking myself out of it !)
Problem is, like so many of you, white would probably look great, but mousy grey certainly not and I don't know how to do any kind of halfway house with henna without risking green hair. Having taken 2 and a half years to grow out a fringe to a long bob which I love, I cannot face a major cut.
Thing I'll stay in the "holding back the years" camp for a few more years yet.