@um9s

Peindre

That's really cool you're trying to learn digital art! It took me weeks to even get comfortable with it. I think you're very smart to tackle colour theory. Tutorials on the subject can be confusing, however. The best way to learn colour theory is to paint from reference. By trying to copy the photo to the best of your ability you're forced to actually see the colours. Because our eyes are easily tricked and it's actually important that you teach yourself what colours you're actually seeing. Instead of watching tutorials, it's much better to actually paint. I know it's boring to tell you to paint from reference, but it is the fastest way to improve your art. Of course you should always draw what makes you happy first and foremost! For how to fix this piece, I think what could specifically be improved is your understanding of form. There is a bit of flatness to your character. Remember that the direction of your brush strokes matter. You should be following the form when you paint. Since the top of the head is round, the highlights and shadows of the hair should also curve around the skull. This also applies to a simpler cel-shaded style like on the right. The highlights and lines still follow the curve of the head. A part of colour theory is understanding values. Values are used to indicate plane shifts. Planes are the surfaces of objects. So for the hair, I noticed you have a lot of value changes. Technically hair is made up of a bunch of tiny strands but when painting it, it is much better to instead create big groups of hair that you treat like one big plane. I think simplifying your rendering style could help you. I included two different rendering styles. It also might be helpful to know that your drawing skill enhances your painting skill and vice versa. You should always be working on both skills simultaneously. Hope this helps and good luck!