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!!
If your hair is dense, it may process faster closest to the scalp but take longer towards the tips. To compensate for the difference in time due to hair density, apply the color to roots (hair closest to the scalp) first and allow it to set for 10 minutes before coloring the rest.