First of all: You have some fundamentals to iron out.
1 Focus on more interesting poses draw your portraits in SOME kind of perspective -- at first it'll be harder, but you'll soon realize you're making it reaaaalllly difficult on yourself to paint a
good portrait just because you're picking a static, ugly pose. No amount of hard work will polish a bad pose/composition into a great painting. Great artists make great art because of great choices.
2 Learn to paint form. Form is simpler than you think. Form is shadow where the form turns away from light, midtone where the form meets light, and sooomeeetimes little highlights where the form reflects the light source directly. You can learn about form
here, and also everywhere else.
Now, specific advice:
Bad news: You've got this muddy, terrible, unappealing shading and rendering going on in every one of these pieces. There are a lot of poorly thought out shadow tones splashing all over the place and it makes you images look like a fuzzy mess.
Good news: Doing it the right way is actually EASIER.
Simplify -- most things in real life, as perceived by humans, are made up of 1 or 2 simple tones and one transition between those tones per shape.
So
SIMPLIFY.
First step, knock almost all of your tones/colors/shades out, and CLEANLY define the forms you need with just 2 or 3 tones per material. While you're doing this,
ZOOM OUT OFTEN. If it looks how it's supposed to when you're zoomed out, then you're doing your job! Dont worry how it looks close up. That's for later!
Second step, you have this nearly cell-shaded base to work with, now how do you make it a 'realistic' rendering!? Well, look at the edges! MOST of the information you as an observer use to understand texture, pattern, and surface material, comes from these transitions. Form is revealed in transitions. Material is revealed in transitions. So paint those transitions! Note the differences between the fur, hair, and different parts of the skin. It looks like a TON is going on here in my gif, but it's actually stupidly straightforward, which is why it's important to go through the steps in order:
I'm picking a color inbetween the two shades i need to blend, and painting it over and around the transition until it describes whatever subtle shape that i'm trying to capture. Sometimes the transition is very big! Sometimes it is very small!The third step is going to be impossible to explain, it's up to YOU. This is where the magic happens, and you start capturing the little details from your reference, or infusing it with your own personal style/judgement. Change pose/materials/colors without regret! Keep your simple base, DONT overcomplicate it, dont be afraid to erase and go back if you do make a mistake, and paint it until it looks rad. Have you ever come back to a drawing after a long time away from it and seen a bunch of things to improve it? Refining a piece is about supercharging that process. Change things even if you aren't sure what's wrong with them! Make it happen!
EDIT: Again. Simplify. Your colors are fine, but your muddy shading is killing them. I'll pick your colors, and lay them in without the overblending:
isnt that better already? (also changed the proportions -- double check those with ref! it's easy to get tunnel vision.)