I posted swatches of Dating a Royal quite a while ago, so in this post I'll show you some comparisons that I have done since then.
OPI Dating a Royal vs. American Apparel Cameo Blue - I absolutely love the AA. The formula is smooth and creamy, and it's really opaque. 2 coats of AA, 3 coats of OPI.

American Apparel Mount Royal vs. OPI Dating a Royal - Mount Royal is a bit closer to DaR than Cameo Blue, but it's a bit purpler.

Essie Mesmerize vs. OPI Dating a Royal - This Essie is quite opaque as well, at 2 coats.

American Apparel Cameo Blue, NYX Girls True Navy, OPI Dating a Royal - These are all kind of similar. Looking at this, I wonder if the NYX Girls is closer to AA Mount Royal. Maybe I should try that later.

Anyways, I'm not sure whether to keep or purge Dating a Royal. I might have to defer the decision for now. I think I am leaning towards purge, since I adore both the American Apparels and like the Essie one as well. I've already purged the NYX Girls. Ahhh... Why is it so hard to purge a polish?! lol!