Thank you!
Kill em!
Go to MAC and ask to use a dark brown lip pencil (not Cork, if they hand you Cork just tell them it's not dark enough) and line your lips and feather it in. Then try on SiS. Apply it more opaque in the middle of your lips and once you get towards the outer part of your lips use your finger to pat and blend the lipstick into the liner a bit. Don't worry about being sanitary because they'll give you a makeup wipe and sanitize the pencil and lipstick. If SiS is sold out at the counter try another light brown lipstick shade that's similar and see if you like the effect of the liner and the light lipstick. Don't wipe your lips when you're done trying it on. Wear that shade home and take lots of selfies.
Btw always wear a full but neutral face of makeup when going to try on lipsticks. I always just wear my favorite foundation, do my eyebrows, put on a neutral blush (MAC Raizin usually), and maybe wear some eyeliner. I think it's important to wear a full face of makeup when going to try on lipsticks until you can visualize what a color will look like even without your full face on. But I wouldn't worry about SiS. I am dark and I love it!
You can see below that the edges of my lips are lined and that's how you control the color and make it not look ashy. I look lighter than I am because of the lighting and I was in a mirror but I'm like a MAC nw45-47. I think SiS fine on it's own but it is also a great base for lip glosses that are brown or have flecks in them. I love pairing SiS with Seducing Sound Lipglass. It also wears well with the NYX Butter Glosses in Gingersnap (to darken it), Praline (for a neutral gloss that doesn't really change the color), and Tiramisu (to lighten it a bit more, it makes your lips look fuller).