@mich2 not entirely related but Andrew Price (Blender Guru) has a cool talk on YouTube called The Habits of an Effective Artist and he talks about taking on the challenge of getting good at drawing in a short time. He is a 3D artist but in general he has a great youtube podcast channel about artist habits and improving skills. https://youtu.be/vM39qhXle4g
The Golden Compass by Philip Pullman is I think one of the best written sci-fi fantasy stories out there. I immediately picked this book when I saw this challenge as it remains one of my all time favorites. Here is my final full cover as well as some closeups for details.
Paul, I agree with Rebecca, there are many types of contrast important to the look of your overall piece (as she mentioned like color temperature). So for example you may want to put cool colors next to warm ones to make them pop. From what I am seeing you are using mainly warm grays throughout the image- and using gray is fine however, transitioning from using a warm grey to a cooler one would help create contrast. So for example where you have the dark blackish brown color on the jacket, you may want to pick a blueish black there for more contrast. You also can consider the colors themselves on the color wheel - if you are using blues, opposite that is orange so again, using these together will create contrast. I would recommend checking out James Gurney's book Color and Light for learning about color theory, he is the master at color. Also, trying a few things while working to check your contrast, First, you can turn your image to greyscale - you may see that you have values ranging from 5-9 on the scale but nothing in the 1-3 area (check out the pic I uploaded for reference).Having that range will make your image pop! So maybe you need a few areas of that lighter value. Working with the scale next to your work is helpful practice sometimes. Next, if you are working digital as it appears from your picture, use your color picker/eye dropper to select different areas - is it all showing you picked different variations of a red or orange, but no blues? Then you are using different tones of the same colors and not varying much color. Last, are you working in CMYK or RGB? I ask this because CMYK is less vivid working in digital (I have attached some photos of this as well) so you may also feel there is a vibrance missing just from this color setting. I hope this helps, and I really enjoy your art!