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!!
Though some people prefer to wait as long as two to three months between reapplying hair dye, a period of six to eight weeks is sufficient enough to reapply hair dye. Permanent colouring dyes contain a greater quantity of Ammonia and Peroxide where as temporary hair dyes contain either none or negligible quantities.