After all the commotion about Chanel Jade, when Riva went on sale I jumped right on that bandwagon. I finally picked up Sky last night so I decided to compare them.
As you can see in this photo, both polishes are pale blue with hidden shimmer. In the bottle Riva looks a touch dustier than Sky.
I actually had a hard time getting Riva's shimmer to show up at all in any photos, but Sky's shimmer just jumps right out front and says "haaaaayyyyyy!"
Here they are on the nail (please ignore my scary dry cuticles, yikes! And also the place where I messed up my middle fingernail!) From index to pinky, I have Riva, Sky, Riva, Sky. I used four coats of each, and to get opacity you're going to need at LEAST three thick coats.
On the nail, under halogen light, Riva is CLEARLY dustier than Sky. You can see here, though, that most of the time you won't be able to tell - they look almost identical on my ring finger and pinky. Both polishes are streaky and sheer-ish, though they build well. As in the bottle, Sky's shimmer is a little more evident.
Verdict: If you're a Chanel addict, get Riva. Otherwise, save yourself some money and get Sky instead. Most people will never know the difference, and they're close enough that even someone familiar with Riva probably won't be able to tell, either.