Udo: Have you tried Garnier HerbaShine? That is the first semi-permanent color I tried and absolutely loved it until this discontinued a shade I was mixing with another shade. The last time I used it earlier this year, the color lasted almost 8 weeks. I couldn't believe it. And, it took care of those pesky grays in my hairline and temple area.
My hair really grabs onto color also, that is why I was mixing 2 shades. Maybe you should try HerbaShine, a medium brown shade mixed with a light brown shade. I liked their ash line to counteract any reddish tones that my hair naturally wants to pull. I tried NI, and while the 2 shades I mixed produced good results, the color didn't last at all on my gray. And, it made my hair more dry compared to the HS which left it feeling very healthy.
Right now, I contemplating Robert Craig color (no perioxide or ammonia). I really wish there were more choices in semi permanent brands and shades.
3B Fine (lots of it), Normal to High Porosity
Low Poo: Elucence Moisture Poo, Renpure Organics
Co-Wash: Tresseme Naturals, Giovanni TT
Rinse Out: Biologe Cond Balm, SS Deeply Decadent Moisturizing Treatment
Leave-In: Oyin Hair Dew, CJ Beautiful Curls,
Stylers: SS Curl Enhancing Jelly, SS Firm Holding Styling Gel, Jessicurl RR and CC, BRHG, Biolage Gelee
Extras: ACV rinses every couple of months