Apologies, I've maybe misunderstood the intention of this thread: surely the point's not to say "x and y are similar" or not, but to say "if you're considering buying x but already have y, it's an inessential purchase, you can use what you've already got"??? Which is subtly different if you see what I mean????
Like; I love my Steamy, my Shimmermoss AND my Aquadisiac, BUT I would say that if you've got Shimmermoss, you've got your "shimmery teal" requirements covered, so you don't really *need* Steamy (which is bluer) or Aquadisiac (which is brighter)...
On a similar vein, if you've got Pink Opal pigment, you don't really need Seedy Pearl or Rite of Spring, or possibly even Lightscapade MSF (I *shock horror* lost my beloved Lightscapade, but have managed to get a not-quite-the-same but similar pearly opalescent highlight by using Pink Opal on my cheekbones.... its not the exact same, but close enough I can't justify paying megabucks on ebay to secure another Lightscapade)
If you've got satellite dreams, you don't really need pro plum (which is similar coloured but DEEPLY not as good)
If you've got Coppering, or Falling Star, you don't really need the other: Coppering is oranger and has a smoother finish (and is probably overall a better shadow), Falling Star's a VERY similar colour but with rosey pink undertones....