I decided to put on my tube of MAC Viva Glam Sia one morning and straight out of the tube, this matte scarlet looks VERY much like a creamier Ruby Woo, although it's a regular Matte, not a Retro Matte. It's also just a hair warmer than Ruby Woo, which has a hint of cool undertones although I still consider mostly a true red. The undertone difference may not be noticeable to a lot of people.
The difference comes when you press your lips together and wear it out in the daytime. The warmth in the undertone comes out more in daylight, and you may - like me - suddenly notice all those tiny little flecks of gold hidden in the matte red formula.
The sparkles are not really noticeable to someone standing in front of you, unless they come really close and squint. But it does give the formula a tiny hint of luminosity, the way velvet sometimes catches the light at certain angles.
A bit of a fun surprise because I didn't look at any press copy or descriptions online. I just bought the tube when I saw it in-store.
For better comparison, here is Viva Glam Sia swatched next to two other MAC scarlets, as well as the new LANEIGE Two Tone Matte Bar in #6, whose two shades blend into a scarlet as well. The camera does pick up on some of the sparks but I almost wish they were a bit more visible on the lips. How cool would that be?
Still, that might make the lipstick slightly uncomfortable to wear. And as it is, Viva Glam Sia is sufficiently smooth for a matte, and goes on dense and opaque. Worthy of hoarding, if you've ever wished Ruby Woo was less dry, and Revlon Really Red was a bit more opaque.