Techmom: I just started coloring my hair a year ago due to some gray in my hair line and at my temples, not much, but very noticeable when I pulled my hair back since I have dark brown hair.
After doing some research, I decided to try a semi-permanent color. I didn't want to commit to permanent since I really didn't want to change my hair color, only hide the grays peeking out. I decided on Garnier HerbaShine. After trial and error to get the right shade, I finally got the perfect mix of light ash brown and medium ash brown. I found that with this mix my hair color lasted 8 weeks (yes, 8 weeks).
I used the natural medium brown and natural light brown when I was trying to decide on shades. But, my hair had a reddish cast that I didn't like at all. I chose the ash since it is cool toned and I don't get any reddish cast. Now, Garnier has discontinued the light ash brown. Just my luck. I have learned also that semi permanent tends to run 2 shades darker (at least on me), that is why I opted to mix 2 shades together.
I have tried Clairol's Natural Instincts (light ash and medium ash) twice now. While it did okay, the color coverage didn't last on my grays at all (maybe 2-3 weeks at the most) and my hair seemed to be drier than usual.
I'm getting ready to try Loreal's semi permanent line now. I have found that I need ash tones to counteract any red that my hair might pull.
I have thought about trying Sally's semi-permanent lines but haven't went that route yet. If I do, I will most definitely mix med ash with light ash. I get the most natural coloring doing this.