You sound like me.
I've been frustrated with this for over 6 months now, and it's driving me crazy....I posted a blurt about it a few weeks ago.
My line sounds similiar to yours. It's never very dark (purplish-bronzy), it's barely 1 mm wide, but it looks like it doesn't go all the way down, most of the time. With a light source behind my thumb, it clearly doesn't go all the way down to the cuticle.
My doctor has looked at it twice, and he's unconcerned by it, but when you research it, the first line is always that such lines should be biopsied (even though the vast majority of biopsies show a benign result) , so right now, I'm trying to decide if I should go and get a second opinion or not. My doctor assures me that he believes it is a stable thing, and he's not concerned at all, both by the color and the presentation.
I've called around for recommendations for second opinion, and my doctor's name keeps coming up at the top of the list for a second opinion.
It's frustrating, to say the least.