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!!
Try to limit washing your colored hair to three times a week and wash with warm water. Don't wash your hair until at least 24 hours after coloring, and if you can, wait at least 48 hours. If you style or swim regularly use a clarify shampoo once a week to get rid of any build up. Add a spoonful of vinegar to a glass of water and rinse through your hair after washing. A vinegar rinse will help close the cuticle and retain your color. Regular treatments such as a hair mask once a week and a hot oil treatment once a month will make sure that your hair is well moisturized and help with fading.