@Blueowl, henna has completely solved his problem. It has been over three weeks now and that one grey hair is invisible. I am attaching a picture. These are just my brows, without brow mascara. With brow mascara, it is even better.

I cannot seem to find the before picture, to show you how that one grey hair stuck out. I will keep looking.

But overall, I am very happy. Of course, it adds cost to the monthly maintenance, but I hated that grey hair sticking out and brow mascara could not tame it or cover it with pigment.

My threader gives me free threading for doing henna. So she does threading and henna in one go, with the threading being free. So that helps somewhat with the cost.

Note that at first it looks very intense and then you take a shower in 24 hours and everything becomes fine. So you cannot do henna immediately before an important event.