don't see "hue shifting" as a techniqu to apply. It's a super simplistic rule of thumb for much more complex concepts which come with color temperatures, light color, reflections, subsurface scattering and much more.

The appaerence of the slime shifted because a milky white as a color behaves different  than putting in a quick hueshift.

That being said I can't see anything wrong with the colors. And the outline also seems fine to me.

But colors are ony one thing and shading and the usage of them is another field. Your shading is just thrown in and doesn't follow the spherical structure of the shape. Therefore it looks really flat.

you could get an image of a waterdrop to get an approximation as a starting point and try to learn the principles. That being said translucent transparent materials are much arder to shade than something solid with a bright color e.g. plaster. Which is the reason why art training usually starts with drawing plaster cast studies.

e.g. google for "water drop on leaf"
Spoiler
Also consider the form of a waterdrop, there should be surface tension, especially connected to the ground

a little edit just with those minor form edits, keeping the exactly same colors and reshading:

first skull was betterTHe reference isn't really well suited for being used as pixel art.
That kind of subtle roomlight just won't work out really readable on low res.Plus the colors don't really help to modulate the shape.
Plus I think there are some proportions you could emphasize in a different way at this small res.

I think the more realistic style you are going for and the resolution don't really fit together. If you go to 150% size or 200% you will have a smaller fight with the placement choices and you cna focus on getting a good drawing.

-you could use atmospherical perspective on the far away horizon to liven up the scene
-you could apply more perspective to the trees
-you could give the battlements a portruded profile
-the contrast of the stones on the top is too much. Introduce another color which ties it together to a single plane
-the front merlons are perspectivically seen to small for the side merlons, move them in the middle
-get rid of that double pixel black line in the stone bricks at the frontplane of the tower

just out of curiosity: is there any specific system you are doing this for or are there any restrictions you set for yourself?

Fixing stuff in order to get it out of the uncanny valley:

-removed "dead body" skincolor
-removed amphibic feeling slimey texture
-fixed the proportions of every feature of the face, all of them were too big
-fixed the positioning of the eyes
-exchanged the male eyebrows with female ones
-edited the right cheekline
-fixed the shape of the lips
-fixed overall values
-fixed the crown of the head / origin of the hair that it doesn't look bald up there
-fixed some things regarding lighting



uncanny valley problems:
all of your parts are too big and too round.
You emphasize texture over light, where no texture should be visible.
Your colors don't model a believable living human face, your colors remind me of the makeup of monsers or "dead skin" gradients, not helpful to portray a beautiful female.
You get bogged down in working on details, while your basics aren't strong enough fleshed out in the sketch to actually paint it in a realistic style.

Focus on proportions and structure, it is definitely missing in this basic sketch you posted. You  currentlycarry all those issues throughout the whole piece and even my edit is twisted in spots, just because i fixed thing by thing,instead of starting from scratch, whcih might have been the better idea.

Ok -  one of firsts backups - to show where I started

GBA:
-Zelda: Minish Cap
-Sword of Mana