The gray stuff that you see means you've boiled the egg(s)way too long (overcooked)..starting the water cold doesnt make a difference at all, it's all about timing really..
Another way to avoid the dreaded gray (it looks green to me though) layer between the yoke and egg white is to take a thumb tack and poke a small hole in the large end of the egg before you put it on to boil. The green is caused by sulphuric gas that is generated as it is boiled. The small hole on the large end allows it to escape during the cooking process.
Like said above, it is also caused by cooking too long. Put the eggs in the pan, cover, bring to a boil, turn off heat and let set for ten minutes. The hangover heat will cook the eggs. This cooking time plus the hole in the large end will result in beautiful yellow yokes with no hint of gray/green.
The key, and I'm a convert, this works no doubt, try it. Use luke warm water, bring it to a boil, as soon as it boils turn it off and leave covered for 20 minutes, not a minute too long... perfect hard boiled eggs. Yellow, not green/grey, perfect.
When i was young, I used to boil for 10 minutes or something. When I read this I called bs, but I tried it anyway. They turned out perfect.