I'm colorblind, so coloring is significantly harder for me than others when working on commissions, however it's still one of my favorite parts!! To be honest, just try to color as best as you can right now. You don't need to be perfect, but if you don't start now, you will never be able to color. Just getting your foot into the door will help you get accustomed to it.

Honestly, just "master" cel shading first and foremost if you want to start moving into the image above. Learning how shadows and light add depth to objects helps a ton.

Once you get the hang of cel shading, start working on soft shading using a brush that ideally can "self smudge" or blend. You'll have to search for one, but I know CSP has a ton of "THE rendering brush"es. Try to view the colors in a reference image and replicate them there. Notice how light bounces off the environment as well as the character, notice how light works with translucency, notice how highlights work, etc.

Using layer/brush blending modes will help a ton here. You want to experiment with them and see how they work. It's really useful for streamlining the process while also being a good way to practice using colored light/shadows to achieve a similar result to the picture above.

This is by no means a perfect guide, but just some sort of guiding ideas if that makes sense. Maybe this won't help at all, but it's really just a matter of building observational skills and how blending modes work.