On the nail, at three coats each, the only difference I could see was in the opacity.
When I went to four coats each, the difference was lessened, though still visible.
I'm not sure why I didn't think to do 3 coats of Blue Moon and 4 of Riva; that would have been closer still. For me, Blue Moon is a fine alternative to Riva, slightly better than Hard Candy Sky because the base color seems a bit of a better match. I wonder if Barry M is working on a Mimosa copy now.