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
Different strokes for different folks. Depending on their learning style, some love Loomis or say Keys to Drawing didn’t help them a bit. Truth is, most artists eventually read them all and use portions from each of them.
My personal reading focused more on the philosophy of art. I wanted to learn the traits and mentality of a successful artist and why they do what they do.
Books by Steven Pressfield:
The War of Art,
Do the Work,
I also re-read The Art Spirit by Robert Henri.
You should check out the book "drawing with the right side of the brain", super critically acclaimed book that is great for learning
also, yes, anyone can draw well with enough practice