My all-time favorite lip liner is Gypsy Rose from Fresh. They have repackaged it into a double sided pencil together with Poppy Plume, but I'm just happy it's still available. I've been using it for at least five years, maybe longer. It's about half shade darker than my natural lip color and goes perfectly with every plum/rose/mauve gloss or lipstick in my wardrobe. It's easy to apply, says put for hours and does it job in keeping top coats in place.
Yesterday I reviewed the other natural color option, a rosy brown lip stain/gloss, so a good match for that would be Dior lip liner pencil in Natural Beige. The color almost disappears on my lips, which is exactly what I need most of the time. It seals the edges and keeps the lipstick firmly intact, and while it's not as soft as the one from Fresh, it does have a slightly better longevity.
There are other pretty colors in this range and I have my eye on the one in Candy Rose. Testing the shades can be a bit frustrating, as they appear darker on your hand than they'd be on your lips (I'm not the only one who first tries lip products on my hand before putting any on my face, right?), but since the Sephora swatches are a bit off you absolutely must try them in person before making your choice.