Important: One thing to note up front here is that not all images actually have any areas in them that are supposed to be neutral gray, and obviously, you can't find what's not there, so if that's the case with your photo, no technique for finding midtone gray will work.
Unfortunately this colour is much more true to blue than green. I now have a soft turquoise but washed the colour out after only 10 minutes. So I'll be surprised if it lasts for more than 2 weeks. Overall I think the Joico colour has great pigment but don't go with the colours you see on the package!!
Typically henna color fades in one to three weeks depending on your skin and how much you wash the part where henna is applied. For longevity of henna color, we suggest less washing of the body part where henna is applied and less use of soap.