Well I'm no professional and I don't really learn so well. So, this is my opinion.
By general stuff I'm going to assume you mean: Shading; Coloring in line art; Correcting line art.
Honestly your work is already far above mine. Your art style isn't similar to mine so what I would classify as 'mistakes' aren't mistakes in your style.
The eyes are a lot longer than I would make mine cause mine tends to be circles and triangles.
I enjoy making shading on the ground when I add BG that are actual places and not just abstract BG cause I don't want it to look like my charters are stickers or just floating in their surroundings. I mess up a lot on this at times.
Although the moon and stars look cool and I love it in your art but if the moon was actually that close to the earth it would be covered in water (I think) But then again your drawings, much like mine aren't placed in the real world (At least I assume they aren't) Point: Think about where you place your characters and why.
The first one looks cool. In my opinion I do feel their body is a bit long (unless that was intended) and at that angle it looks like they're just going to flip over and land on their back.
I can't give you any tips on colors. If anything I should be asking you for tips cause I still struggle with colors no matter how much I drill color theory into my brain. And so far all your colors look good to me.