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!!
How Long to Leave Hair Color On. How long you should leave hair color on depends on the directions from the manufacturer, but usually it is about 30 minutes. Make sure you know how long to leave hair color on with tips from a professional hair stylist in this free video on hair highlighting. Part of the Video Series: Hair Highlighting.