Your new version looks a lot better. I'm happy to see someone who's positive about learning, so I made an edit to demonstrate more stuff
(one on the right)
This isn't supposed to be 100% so some things might be a little screwy. Like, I made the whole right half too busy (yours has negative space there, so it's easier on the eyes)
Anyways I think it'd be helpful if I tried to go as far in-depth about what I did as I can, so...I'm going to do that. Please forgive my habit of writing an overwhelmingly huge amount.
I've heard a lot of artists say, "draw what you see, not what you
think you see!" It took me a really long time to understand what that meant, but basically, there are a lot of types of observations that you have to train yourself to notice through practice. I think hearing me describe some of my thought process when I made this edit might help guide you in the right direction.
Don't worry about absorbing it all at once, but slowly try to add more things to your considerations. These types of observation are important for pretty much all areas of art, from drawing objects to drawing your own characters consistently.
I used the "pick color" tool to take colors from her hair on the official art. I didn't USE those colors in my sprite, but I did it to get a general gist of what kind of hues and such were in her hair, since they're pretty unorthodox. Then I just took stabs at it until I was satisfied.
As a note, I think that colored outlines should usually be quite dark, or else a lot of detail gets blurry and lost (the bow is hard to see properly) or the parts with those outlines don't feel like part of the whole.
When I was in the middle of writing this post, I compared my earlier version (on the left) to yours, and that made me realize that my hair colors were causing a lot of readability issues with the skin. My re-attempt is on the right. There are still some issues with the arms, so my next step would probably be to mess with the wrist bands because the pink and orange next to each other are confusing, but i'm still not completely sure how i'd go about fixing it.
I exaggerated the curves on her tail...drill...things a bit like the art did it. They're probably her most obnoxious feature, so it's important to try to get accurate with them. Exaggerating curves can be a great way to generate interest in a piece, and Nui's design uses that concept.
I re-proportioned her head. The eyes still aren't exactly Nui, but the face is taller now (which made the forehead proportionally a bit smaller), so the eyes are at least given a bit more prominence.
Uhm...the skirt is difficult to figure out and I don't feel like doing it so I just did whatever, hehe. In the artwork the skirt forms really strong curves that are kind of like a super flared out ^ shape, so it'd be ideal to replicate that. Mine is kind of a ^ shape without the "super flared out" part. My curves along the sides look rather convex, where the art says they should be concave, and I didn't really manage to put ruffles in the top two tiers...Getting these details can be hard! The bottom tier of the skirt lends the most to the silhouette, though, so i tried hardest on that.
Anyway, whew, that's all I have to say for now. I hope that wasn't too overbearing, ahah