What does dye release mean? I followed the instructions on the container.
Dye release = letting the henna and liquid mix sit until you get a good stain from the paste. How long depends on the henna, usually. The Jamila I get from the Indian grocery takes about 12 hours. I usually prep mine, freeze it, and then thaw it in warm water before I use it (double bagged). I seem to get better stain that way.
I leave the henna on my head for 2-3 hours. It takes a day or so for the color to fully develop. When I started doing henna again, it took about 3 applications before I got good color depth (but I was blending bleached hair with natural). You won't see a lot of difference on dark hair unless you are in full sun, but your greys should cover after a couple applications.