Drawing on the Right Side of the Brain: The Definitive, 4th Edition
that's a very good start.
Start with this: https://toptalkedbooks.com/amzn/1585429201
you might wanna checkout these two books too while you are at it -
Drawing on the Right Side of the Brain: The Definitive, 4th Edition ...
You Can Draw in 30 Days: The Fun, Easy Way to Learn ... - Amazon.ca
A bicycle is an incredibly simple visual form. You can doodle one in about five seconds. They're not rare or unusual objects and they're relatively homogenous. Nonetheless, most people have never actually seen a bicycle. They've looked, but they haven't understood its form, they haven't decomposed it into lines and shapes. They know that it has two wheels, a chain, a saddle and some handlebars, but they've never actually noticed the shapes that join them together.
This describes the premise of a left brain and right brain in layman's terms. For a whole book on the topic go read Drawing on the Right Side of the Brain . It explains the idea of symbol drawing more in depth and provides a bunch of exercises to go with it. Some exercises require you to have a see-through pane (like glass). I've read that some people think it's okay to just skip them, but I did those too. You can work through the book in about a week of 2 hours each day and hugely improve your life drawing skills. You can see some before/after pictures here and I can attest you that the progress depicted is accurate.
Overall, I think that the author uses way too many words to describe the concepts presented, but there simply is no other book (I've heard of) that describes the same concepts.
Personally, I'm more of a learn-by-doing person. I would suggest looking for some local art classes. Color theory sounds like it would be the most useful thing for you.
If you do end up buying a book, try to find one that has lots of exercises, and basically treat it like a class. Don't just read all the way to the end of the book in one sitting. Read a chapter, do the exercises from the chapter, and then wait a day or two before you move on to the next chapter.
The book Drawing On The Right Side Of The Brain is a good example of what you should look for. It's obviously tempting to say "I don't need to know about drawing!" when you work primarily digitally, but learning drawing really teaches you a lot about the fundamentals: perspective, composition, light and shadow, etc. To re-use my musician analogy, pretty much all composers start by learning to play an instrument (usually piano) before they start writing music. You don't need to be an expert, but it's very important to understand the fundamentals.
Oh, also apparently the same author has a book on color theory , but I haven't personally read it. Might be worth a try.
Its been a long journey but I am definitely still learning. The book Drawing with the Left Side of the Brain has a lot of really decent activities that can get you started: https://toptalkedbooks.com/amzn/1585429201
The problem is that it is like any other skill in that the time involved getting to a level you are happy with can take a LOT of time and forces you to neglect other things you could be doing with your time.
I only mean this as constructive criticism and not to hurt your feelings, but: everything. I think you should take a step back from color and painting, and worry more about sketching and strengthening your foundations.
You can still do this digitally, and I'm by no means telling you to stop painting! However, foundations (especially in your case: edges, lines, values, and form) are absolutely the to improving.
Edit: I realize I didn't really give much advice in my post so I wanted to come back and actually give you something useful to work with. :P
Start smaller - much smaller. It looks like you're working from the image of another artist, which is a great challenge, but you're putting the cart before the horse. I'm not sure what your visual skills are like right now, but I'm a strong advocate of Betty Edward's Drawing on the Right Side of the Brain . The skills you'll learn from that book are invaluable and expose you to a wide range of techniques and theory that will really help you hit the ground running.
I'd also suggest not getting caught up on one image. There is a lot of improvement to be made, and the best thing you can do as a young artist is to be done when you're done and move on to the next piece. I still get caught up looking at completed work and drive myself crazy wondering what I could have done differently. The truth is that experience comes from mileage, and you've just got to make that journey.
This is a good beginner's sketch of a character.
I'd recommend that you find a drawing critique subreddit if you want more in-depth recommendations, but I can provide a bit of a critique.
I think the biggest area to focus on improvement would be the proportions of your figure. Things generally feel a bit "off" in a lot of places. For example, the shoulders aren't exactly right and the arms are a bit awkward. One of the benefits of doing a pencil sketch is that you can do rough shapes for those elements until they feel right and then revise the drawing. Revision is one of the big benefits to pencil sketches, in my opinion. This doesn't look like you have done much revision at all.
I think the detailing is good, but because it seems like you started with the detailing, the underlying structure is a bit wonky, and it's difficult to appreciate some of the details.
Some of the shading and texturing seems a bit inconsistent. For example, the hands and the shoulders don't seem to mesh.
Do you have any formal art training? You might try going through Drawing on the Right Side of the Brain or something similar.
Overall, I think that this is a promising sketch, and that there are certainly ways to improve; if you have any questions, I tried to talk to you in discord, or you can reply here.
Drawing on the Right Side of the Brain