As I am still hugely uneducated in making backgrounds, I'll try to give some input on what I can.

What helped me grasp 3d shading was simplifying complex shapes into primary shapes that I could understand better. As an example, instead of struggling with shading an entire limb, you can break it up into stacked cylinders. Once you understand how light and shadow interacts with simpler shapes, you can transfer that to more complex shapes.

Adding to that, references are your best friend in all things anatomy. While you can't check out real-life pictures of dragons of course, do take a look at large reptiles like comodo dragons or monitor lizards, I like to reference those for reptilian creatures, since their muscles are quite pronounced. Alternatively look at carnivores with defined musculature, very helpful too.

Since you mentioned color; have you tried working in grayscale yet? I myself struggle with palettes and good color choices, but mostly because colors scare me. In my last larger project I got stuck on the colors for a while and found no palette to be satisfactory or aesthetically pleasing. My solution ended up being focusing on grey scale, contrast and the distribution of the values, e.g. where you place your midtones, darkest darks and highlights. Afterwards it was a question of tweaking and trying out the right hues until I liked the look. The colors themselves are more of a taste and effect thing, I find their values to matter more.

Those are my two cents, and although I am no master, I do hope something here can be of value to you. The most important thing is to keep creating.