Greys from hell!!

Hi,(again),I just read about Maria's hair I have the same problem, color shampoo's or extend color shampoos refuse to grab my greys( 40% grey aaaaahhhhh), it is getting to the point I have to re-touch my greys with a permanent every 2 weeks & use a really soft semi in between that 2 weeks, I'm going mad, my hair is going through a crazy growth spurt as the weather is so hot, I've used every professional tint on the market yet I still have this problem& I don't want to ruin my hair, but as it is so dark I can't get away from coloring it, can anyone help me please!!.All The Best,Simone.(C). :)