Heya!

From what I've seen, rn you've got a pretty solid eye for design, the tail on 'orange cat hoodie girl' and 'poseidon' exhibit curvature to great effect (I take ages on those types of things lol)

As a few others have pointed out, colour is certainly the most immediate thing that can take your skins to the next level. rn your colours are a lil flat - both a) in the sense that there's not a whole lot of contrast between each colour and b) that they are bland as a result of low contrast

Now your next steps regarding colour will depend on what you want to achieve and how much you know about colour theory

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If you're continuing brighter skins like sunboii, contrast between each colour of the ramp will only come from hue and saturation



In the sunboii skin, there's not a whole lot of difference shown between each colour of the yellow ramp (I did notice some very slight changes to saturation and hue)

This next example from St Lee has a lot more contrast, by comparison



In order to get a nice ramp like that, kick up the difference in hue and saturation between each colour.

On the the other hand, if you want to use a wider range of brightnesses, then you get to play with hue, saturation and value (aka brightness)

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You've already got a solid foundation in shading and design; couple that with some neat colour choices and you'll really shine : D

the_soup has a remarkably insightful and concise tutorial on how to hue-shift and vary saturation.
I also wrote a deeper explanation as to why those techniques are important, which might help you along too : )

lmk if you need anything explained, I might've miswrote something horribly at this time of night

What anonpmc2649631 said is great advice. Things I would add:
-Make sure you are comfortable with the editor you use. Knowing how to use it well, and and easily navigating it makes it a lot easier to create something good.

-Saturation. Saturation is really important, I used to make the mistake of having desaturated colours. This can look dull, when you really want you skin to stand out as its own thing.
  I normally use a less saturated tone for skin, and for some shades of hair.

-Use examples from life. Skins and art follow a lot of the same ideas, so shading tips for art can be a great help. Knowing where to put light, and where to put shade helps define a skin

-Step out of your comfort zone a little. It helps you grow into more pleasing looking skins. I used to only do human, teen female skins. I've grown a lot more, and that makes even those skins now look better. New things help expand skill.

-Colour theory and hue shifting. It's a pain to learn, but its really worth it. It makes colours look so much better!

Those are the main tips that helped me, so hopefully they can help you (and others!)

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okay, i'm not the greatest expert in skinmaking (pretty obvious judging from my skins), but i have some advice things that i try to follow and they might help you, too. I try my hardest to follow anatomy when placing clothing and shading the clothing/body. some references I use are this skin or this other skin. Another thing that I did a lot in the beginning was watch quite a bit of tutorials. elfie_ (link to playlist of tutorials here) and Beanie (another link to playlist of tutorials here) were the main skinners that I really learned from. My last piece of advice is to keep working on it! No, really. You aren't going to get any better by procrastinating, feeling bad about your self, and/or avoiding skinning altogether (i know from experience, i really need to work on that).

I recommend trying more shading, thats what mostly I think gets peoples eyes.

i am on mobile right now and it is hard to type a response right now, but when i can get to my computer i might be able to help.