I haven't used Naturtint but I can comment on the bolded. What leads you to believe that RC Color is complicated? It's very easy to use, mixes with water alone, no smell or harsh chemicals.
And as far as grey coverage - my sister and I can attest to the fact that it works (as well as many curlies here)! You can see my pictures in my fotki, and I can tell you there is quite a bit of grey in my crown especially. In fact, you can see it in some of the pictures as they were taken when I needed a touch up.
LMK if you have questions about it, I can help.