I chose Imperial Purple because of the coppery highlights I could see in the shimmer.
On the nail, the copper wasn't immediately visible; it just looked like a plummy wine color (not what I call purple, exactly).
At an angle, though, the copper did show up, turning the plummy wine deeper and richer. It's like a really, really subtle duochrome, and I like it a lot.
I also like the glow Imperial Purple has in low light:
Saturday Night Shimmer is a dusty mauve with a purple flash in the shimmer, along the lines of Essie Demure Vixen. I'm not sure what this has to do with Saturday night—it seems quite office approriate, even for a conservative workplace. Saturday night is much more flashy in my head (not in my reality, though; many times that's when Mr. Karen and I do our grocery shopping since the store is never crowded).
The purple flash in the shimmer is on the shy side, but it does show up more in low light.
I had no issues with application. Imperial Purple was two coats and Saturday Night Shimmer was three (no topcoat on either). I don't think I'd pay full price, but on sale and/or with a coupon I can see myself getting more colors.
UPDATE 07-Mar-2013: You can see all the colors on the Kohl's Elle page.