Second, let me apologize for how crappy these photos are. I say this way too much--these pictures don't do this polish justice!
MAC Cosmetics "Formidable" is a metallic purple shimmer with a slight blue duochrome effect. It's not strong, but it's there, especially in the right light. My photos show three thin coats. Just using one coat it was the color of grape jelly. Two to three coats produces the gorgeous dark purple you see in the photos. This color is such a different shade of purple than I'm used to seeing, and I fell in love with it when I tested it. Formidable is from MAC's "Venomous Villains" Collection, which was released 2 years ago in 2010. The other two polishes in this collection are called "Bad Fairy", which I'd like to get, and "Mean & Green", which is a dupe for Orly "Space Cadet" & Hard Candy "Beetle" (I have Beetle).
I got this polish at Nordstrom's, and it's pretty expensive... but it was my birthday, lol!
This unfocused photo with flash shows the pretty shimmer.
You can see both the reddish-purple and the bluish-purple in this shot.