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!!
Use a hot oil hair treatment at least 3 days before coloring to condition and prepare your hair strands to take on your hair color. Avoid washing your hair the day of or right before coloring your hair (try to do it the day after your hot oil treatment) so that you don't wash away the natural oils in your hair. They will help you to achieve a more even result.