Using Susanne Russo's (Just Cockatiels & Mousebirds & More) photos, here are photos of normal grey cockatiels.
Since Poptart's grey feathers are not as dark as a normal grey, we can safely assume that she has a dilute mutation. Cinnamon is the most common out there and is like a normal grey but only lighter. Chicks are born with ruby red eyes that darken as they mature. Some cinnamons may be a lighter grey while others may actually have a brown wash to their feathers.
Poptart's back feathers are both grey and clear. Well, more like clear with grey outlines. This is an indication of the pearl mutation. Visual male and female pearls are born looking the same, but males, as they mature, will lose their pearls and look like normal males. (some exceptions out there! particularly in pied birds!)
Although the patterns of pearls can vary.
However, Poptart also has competely clear feathers (stomach, back and tail?). This is an indication of the pied mutation. Pied feathers will be yellow (or white, in whiteface birds) and will have no patterns on them.
If you want to learn more about the mutations with wonderful descriptions of them, Susanne Russo's website is great for that!
Oh my Poptart is a real cutie! I have to say after seeing some of these fancy colors I'm liking cockatiels more and more You didn't by chance get Poptart and sunshine flea market did you? That's where I found my Spikey.