OK, so we need to determine if you like the red/warm tones your hair pulls (neutral or gold) or want an ash/cool shade. My hair pulls red too but I like a bit of warmth. I use Golden Chestnut (gold base) on my length and usually my roots, it turns my grays into warm reddish-brown highlights.
You can see it in my pics (link with avatar, pw in sig).
RC Color does fade, but I find it does so slowly and you don't get a line of demarcation because there is still color underneath (natural). Although you may still find some fading and brassiness until the old color grows out.
Maybe this one would be good to start:
Since the colors tend to go darker, this would probably give a medium brown. If you used Light Chestnut, your hair will be dark brown.
Is your hair darker than mine or lighter?