Issues with hair color?

Hi All, I used to color my hair using chemical hair colors containg amonnia and all. Before 2 months, I have switched to Dr. Urjita Jain's product called as Greynil (black tone). It has all the natural ingredients like henna and indigo powder. I applied it almost every week since one month. But I am unable to get proper results. My white hair are not getting covered completely. After rinsing the Grenil application , I always wonder whether my hair color has changed or not. One of my cousins suggested to use Mahuri henna and he said it covers the greys very well. I had also used it few years before and the coverage was pretty good. However, we need to rinse it off only 30 minutes after applying it. This fast developing of color brings doubt in my mind that it has some harmful chemicals in it (though the packet says it's natural). One more issue with Mahuri is the black color is too dark. It's available in brown shade also but that's bit light for me. Anybody has tried mixing black and brown shades of Mahuri and got good results? Is it safe? Can anyone suggest any other natural remedy/product that will not harm my hair and will also give a good grey coverage? And I dont want to apply it every week (that's too frequent and each weekend I have to give time :(). Please help!! I will be very thankful to you :)