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!!
Hairstylists choose a toner that is on the opposite side of the color wheel from the unwanted tone. If your hair color is too red, a green toner is used; if your hair is too orange a blue toner is used; and if your hair color is too yellow, a purple toner will neutralize the unwanted yellow tone. Toner can last an average of 4 to 8 weeks, depending on how well you take care of your hair color investment. Some toners last longer. When toner fades away, lighter warmer color is often exposed. Toner can be reapplied at the salon when faded, if desired.