I think I can help a bit!
I'm just going to give some quick tips for cat anatomy first, then I'll get on to the colouring. For a first attempt it's very good though.
- Cats have much smaller muzzles, here's a picture of my cat for reference:
https://i.postimg.cc/N0fGVrFv/F1-D116-DF-6963-43-C1-A3-FB-2981-B2-CBA2-C7.png
And the muzzle starts to go more inwards from the nose, it isn't just a straight line down down then back towards the face, if that makes sense?
- The ear is much too big, but it doesn't look bad if it's part of your style.
- There's no definition for the paws, now I'm not great at paws but googling some references will help here. Cats need toes too!
Colouring:
Now just to note, this may not be the best way of colouring, this is just how I do it and you can take advice from it or completely ignore it if you want.
Your fur problem is actually very common, the fur is flat and while it has some direction, does not look 3D, shaded or like fur really.
Now, when I just finish a sketch I usually make a base by using a lasso tool and going around it and filling it in, then I go back and add finer details with the lasso tool like tufts of fur, or claws. But this makes it harder to achieve a natural smoother looking appearance so I would probably not do that. But this is one method you could try.
Anyway, once you have a base, with any method you choose, make a clipping mask above it and whip out an airbrush. The airbrush, in my opinion, is great for plotting down colours! This way the colours blend in together more instead of using harder brushes. Now, the colours you want should not be the colours of the markings, and you should not use the lightest white or the darkest black at all, except for maybe adding tiny details (with white you could do some very small highlights but you don't necessarily need such a bright white to give a nice effect).
So make the colours noticeably darker than they're supposed to be, but not so dark that putting the actual colour on top will look weird.
Get a brush that isn't sharp, and is preferably blended on the ends of the strokes (I'm bad at describing brushes lmao). I prefer softer looking fur. Especially for a cuter scene, like this cat. Start to add fur strokes, make them bigger and make sure you include direction. If there's a certain point where the fur all goes off in different directions, make the fur go in all different directions and leave a tiny tiny slit where you can still see the 'skin' or base colour.
Then just keep layering the fur, making the fur darker in areas of low light and making it lighter where there is light. If the light is strong, at the very end you can highlight around the edge of the cat if the light is behind and also make the whiskers white to catch this light etc.
As you layer, make sure the main colour is the actual colour of the markings. And don't over do it so you can no longer see the base colour, there's a reason why it's there, it helps with depth.
Also, make the fur imperfect! Let it run out into other markings, exceed the base lines. No cat is perfectly smooth!
And don't be afraid to use contrasting colours. It really helps make your image pop :)
Summer 
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